'Miniature Circuit Breaker' (MCB) is the advanced form Remember SI unit
of a fuse. It is used to protect the household wiring Physical Quantity
from overloading or short circuit. MCB switches off ampere (A)
watt (W)
the circuit within a fraction of a second in case of short Current (I) volt (V)
circuit or overloading. The MCB can be reset when the Power (P)
fault has been corrected.
Voltage (V)
Fig. 10.15
Reasonable fact-4
M.C.B. is more useful than fuse.
MCB is the advanced form of fuse which protects the household wiring from
overloading or short circuit where as fuse protects the electric appliances in the circuit
from getting damaged. Fuse requires the wire to be exchanged but MCB does not
require the wire to be changed. So, MCB is more beneficial than fuse.
Selection of a Fuse
When a short circuit or overloading takes place, a large amount of current passes through
the fuse. It causes heating of the fuse and ultimately the fuse wire melts and breaks the
circuit as shown in the given figure.
Fuse wire melts and breaks the circuit
Fig. 10.16
Reasonable fact-5
Fuse is used in household wiring system.
A fuse breaks the circuit by melting itself when the current exceeds the safe value.
Hence, it prevents the electric appliances in the circuit from getting damaged due to
overflow of electric current. So, fuse is used in household wiring system.
PHYSICS Oasis School Science and Environment - 8 145
When the fuse wire breaks the circuit by melting itself, electricity supply is automatically
cut off before any damage due to the overflow of current.
The fuse wire used in a particular circuit must have proper thickness. The thickness of the
fuse wire should be such that it is able to withstand only a little more current than drawn
by the household circuit. Fuse wires are rated as 5A, 10A, 15 A and so on. It is clear that a
10 A fuse wire will be thicker than a 5A fuse. Before the selection of a fuse wire, the current
through the electric circuit must be calculated by the following formula.
Current (I) = Electric Power (P)
Voltage (V)
∴I = P
V
After calculating the value of current, the rating of the fuse should be slightly more than
the current flowing in the circuit. Commercially available fuses are 3A, 5A, 10A, 15A, 30A,
etc. But nowadays fuses of different rating can be found.
Differences between Fuse and MCB
Fuse MCB
1. A fuse is a thin wire made of lead (37%) 1. MCB is advanced form of fuse which
and tin (63%) which is used in electric switches off the circuit within the fraction
circuit. of second in case of overloading.
2. It needs wire to be exchanged. 2. It does not need wire to be exchanged.
Reasonable fact-6
Electricity is a very useful form of energy.
Electricity is one of the key source of energy. Electrical energy is required to operate
large number of electrical appliances like computer, television, radio, calculator,
refrigerator, etc. So, electricity is a very useful form of energy.
146 Oasis School Science and Environment - 8 PHYSICS
SuMMarY
• Electricity is a form of energy which is produced due to the continuous flow of
electrons through a conductor.
• The devices from which we obtain electricity are called sources of electricity.
• A cell is a device which converts chemical energy into electrical energy.
• The cell which converts chemical energy into electrical energy and cannot be
recharged is called primary cell.
• The cell which can be used again and again after recharging is called secondary
cell.
• Dry cell consists of a zinc container and a carbon rod with a brass cap.
• The AC circuit made in the industries, factories, houses, etc. is called domestic
electric circuit.
• To operate any electrical appliances, we need two connecting wires. They are
live wire (L) and neutral wire (N).
• A fuse which is connected before the kilowatt hour (KWh) meter is known as
corporation fuse.
• The main switch box is made up of iron and a connection is made with the iron
box (main switch box) to the earth by a conductor which is known as earthing.
• The equipment which are operated by electricity are called electrical equipment.
• The electrical appliance that converts electrical energy into heat energy is called
electric heater.
• The electrical appliance that converts electrical energy into sound energy is
called electric bell.
• A fuse is a thin wire made up of tin (63%) and lead (37%) having low melting
point and high resistance.
PHYSICS Oasis School Science and Environment - 8 147
exercise
1. Choose the best answer from the given alternatives.
a. Which of the following liquids is used in a simple cell?
i. distilled water ii. sulphuric acid
iii. hydrochloric acid iv. salt solution
b. Which of the following chemicals is used to remove polarization in simple
cell?
i. potassium chromate ii. potassium sulphate
iii. sulphuric acid iv. potassium dichromate
c. Which of the following chemicals helps to remove polarization in a dry cell?
i. manganese dioxide ii. ammonium chloride
iii. ammonium sulphate iv. manganese oxide
d. Fuse wire is made of ................................. .
i. lead and glass ii. lead and carbon
iii. lead and tin iv. tin and copper
e. Which of the following equipment converts electrical energy into heat
energy?
i. heater ii. electric bell iii. mobile phone iv. radio
2. Tick (√) the correct statement and cross (×) the false one.
a. In a simple cell, zinc plate acts as a negative electrode.
b. Local action does not take place in a dry cell.
c. Filament lamp converts electrical energy into light energy.
d. Fuse is made of lead and copper.
e. The full form of MCB is miniature circuit breaker.
3. Fill in the blanks with appropriate words.
a. Copper plate acts as a ................... pole in a simple cell.
b. ................... is used to remove polarization in a simple cell.
c. Live wire is also called ................... wire.
d. Immersion heater converts .................. energy into .................. energy.
e. Fuse is made of .................. and .................. .
148 Oasis School Science and Environment - 8 PHYSICS
4. Answer the following questions.
a. What is electricity? Name any two sources of electricity.
b. What is a cell? Name two types of cell.
c. What is a simple cell? Write down its major components.
d. What is polarization? How is it removed?
e. What is meant by local action in a simple cell? How is it removed?
f. What is a dry cell? Why is it widely used?
g. Write down the uses of dry cells.
h. What is electric lamp? Why is electric heater used?
i. What is a fuse? What is it made of?
j. Why is a fuse used?
k. What is MCB? Why is it called an advanced fuse?
l. Name any five electric equipment that are used at your home.
m. What is electric bell? Wrie down the major use of electric heater.
5. Differentiate between:
a. Simple cell and Dry cell
b. Polarization and Local action
c. Fuse and MCB
6. Describe the structure of a simple cell with a neat and labelled figure.
7. Describe the structure of a dry cell with a neat and labelled figure.
8. How does a fuse work? Describe in brief.
PHYSICS Oasis School Science and Environment - 8 149
11UNIt Estimated teaching periods : Th Pr
9 1
Matter Moseley
Objectives
After completing the study of this unit, students will be able to:
• describe the structure of an atom and state the properties of proton,
neutron and electron.
• introduce periodic table.
• define valency and find out the valency of the first 20 elements on the
basis of their electronic configuration.
• define atomic number and atomic weight and find out the number of
proton, neutron and electron on the basis of atomic number.
• define moleculer weight and calculate the molecular weight of some
common compounds.
• define chemical reaction and equation and express some simple
chemical changes into word equation and formula equation.
Course of Study
• Structure of an atom (proton, neutron and electron)
• Electronic configuration of the first 20 elements and their valency
• Introduction to periodic table
• Molecular weight
• Simple chemical reactions (word equation and chemical equation)
Points to be Focused/Questions to be Discussed
• What is an atom? What are proton, neutron and electron?
• What are atomic number and atomic weight?
• What is electronic configuration?
• What is valency of an element?
• What are molecular formula and molecular weight?
• What is periodic table?
• What are chemical reaction, word equation and formula equation?
150 Oasis School Science and Environment - 8 CHEMISTRY
11.1 Introduction
A variety of substances having mass and volume are found in our surroundings. These
substances are called matter. Examples: Brick, stone, air, soil, water, plastic, etc. Anything
which has mass and volume is called matter. Matter can exist in three different physical
forms, viz. solid, liquid and gas. Among a variety of matter, some are simple and others
are complex. We can decompose complex matter into simple form. The substances that
can be broken down into simple substances are called compounds. Sodium chloride
(NaCl), water (H2O), limestone (CaCO3), carbon dioxide (CO2), etc. are some examples
of compounds. A compound is a substance made up of two or more elements (atoms) in
a fixed proportion by weight. Compounds are formed by chemical reaction between any
two or more elements. The smallest particle of a compound is called molecule.
The pure substances that cannot be broken down into two or more simpler substances are
called elements. Iron (Fe), copper (Cu), gold (Au), hydrogen (H), sodium (Na), carbon (C)
are some examples of elements. Elements are the pure substances made up of only one
type of atoms. All the atoms of an element are identical but the atoms of different elements
are different. Elements cannot be decomposed into any more simpler substances.
Gold cannot be broken down into any simpler substance. So it is called an element. About
118 elements have been discovered by scientists so far. Among them, 92 elements are
found in nature and remaining 26 elements have been prepared by scientists in laboratory.
11.2 Atom
Elements are made of tiny particles. These tiny particles are – Shell or orbit
called atoms. An atom is the smallest particle of an element
which can take part in a chemical reaction. All the atoms of + Nucleus
an element are same but the atoms of different elements are + Proton
Neutron
different. For example, all the atoms of hydrogen are similar – Electron
but the atoms of hydrogen differ from the atoms of silver or Fig. 11.1: A neutral atom
sodium or copper. The atoms of all 118 elements differ from
each other. Atoms may or may not exist freely in nature.
For example, atoms of helium, neon, argon, etc. can exist freely in nature but atoms of
hydrogen, sodium, oxygen, etc. cannot exist freely in nature.
11.3 Molecule
A molecule is the smallest particle of an
element or a compound which can exist freely. HH OO
The molecule of an element consists of two or
more atoms of the same kind. For example, a A hydrogen molecule An oxygen molecule
molecule of hydrogen element (H2) consists of Na Cl HOH
two atoms of hydrogen. Similarly, a molecule
of oxygen element (O2) consists of two atoms A molecule of sodium chloride A molecule of water
of oxygen. Fig 11.2
matter /ˈmætə(r)/ - anything which has mass and volume
molecule /ˈmɒlɪkjuːl/ - the smallest particle of an element or a compound that can exist freely
CHEMISTRY Oasis School Science and Environment - 8 151
The molecule of a compound consists of two or more atoms of different elements. For
example, a molecule of sodium chloride (NaCl) contains one atom of sodium (Na) and
one atom of chlorine (Cl). Similarly, a molecule of water (H2O) contains two atoms of
hydrogen (H) and one atom of oxygen (O).
Differences between atom and molecule
Atom Molecule
1. An atom of an element is the smallest 1. A compound is made up of two or more
particle of that element. types of atoms.
2. It can take part in chemical reaction 2. It is the result of chemical reaction.
11.4 Structure of an Atom
An atom is the smallest particle of a matter which may or may not exist freely in nature.
An atom is made up of three fundamental particles, i.e. proton, neutron and electron.
These particles are also called sub-atomic particles.
–
– Nucleus Reasonable fact-1
++ Proton (p+) Atoms are electrically neutral.
– ++ – There are equal number of positively
charged protons and negatively
++ Neutron (n0) charged electrons in an atom. So,
atoms are electrically neutral.
Shell or orbit
–
– Electron (e–)
Fig. 11.3 Structure of a carbon atom
Among three fundamental particles of an atom, protons and neutrons are located at the
centre of the atom, i.e. nucleus whereas electrons revolve around the nucleus in elliptical
orbits called shells. The structure of a carbon atom showing the position of sub-atomic
particles is given in the fig 11.3.
Protons
Protons are positively charged sub-atomic particles located in the nucleus of an atom. The
mass of a proton is equal to the mass of a hydrogen atom. The mass of a hydrogen atom
is regarded as 1 atomic mass unit (amu). A proton is denoted by the symbol p+. The mass
of a proton is 1837 times larger than that of an electron.
Neutrons
Neutrons are chargeless sub-atomic particles located in the nucleus of an atom. The mass
of a neutron is equal to the mass of a proton. A neutron is denoted by n0. The mass of a
neutron is 1837 times larger than that of an electron.
152 Oasis School Science and Environment - 8 CHEMISTRY
Electrons
Electrons are negatively charged sub-atomic particles that revolve around the nucleus in
pelrloiptotinc aolr o ar bnietsu tcraolnle.d T shhee mllsa.s Ts hoef amna eslse octfr aonn eisle acbtroount i1s 8v13e7r yth l oesf sa apsr cootomnp oarr ead n teou ttrhoant .o Af na
electron is denoted by the symbol e–.
Differences between proton and electron
Proton Electron
1. Proton is the positively charged of an 1. Electron is negatively charged
atom. sub-atomic particle.
2. It is located in the nucleus of an atom. 2. Electron revolves around the nucleus of
an atom.
11.5 Atomic Mass Unit
The mass of an atom is extremely small. So it cannot be measured in kilogram (kg), gram
(g), milligram (mg), etc. Therefore, the mass of an atom is expressed in amu, i.e. atomic
mass unit. The mass of one hydrogen atom is taken as 1 amu. The mass of a proton is
approximately equal to that of one hydrogen atom. So, the mass of a proton is about 1
amu. The mass of a proton is equal to that of a neutron whereas the mass of an electron
1
is about 1837 th of that of a proton. About 6 × 1023 protons form 1 gram mass, i.e. 1 gram
= 6 × 1023 amu.
11.6 Electric Charge
Out of three sub-atomic particles, proton and electron contain electric charge. Protons
contain positive charge and electrons contain negative charge. But neutrons do not have
any charge. They are chargeless sub-atomic particles. In SI system, electric charge is
measured in coulomb. It is denoted by C or Coul. One coulomb charge contains about
6.25 × 1018 electrons.
A neutral atom consists of equal number of protons and electrons. Therefore, an atom is
electrically neutral due to the presence of equal number of protons and electrons having
opposite charges. The comparative study of protons, neutrons and electrons is tabulated
below:
S.N Sub-atomic particles Symbol Location Mass Charge
1. Proton p+ Nucleus +
2. Neutron n0 Nucleus 1 amu
3. Electron e– Shell or Nil (0)
orbit 1 amu –
1 amu
1837
neutral /ˈnjuːtrəl/ - without any charge, chargeless
CHEMISTRY Oasis School Science and Environment - 8 153
11.7 Atomic Number
Atomic number of an atom is defined as the number of protons present in the nucleus of
that atom. In a neutral atom, the number of electrons is equal to that of protons. So, atomic
number can also be defined as the total number of electrons (e–) present in a neutral atom.
Atomic number is denoted by 'Z'.
Atomic Number (Z) = No. of p+ = No. of e– in a neutral atom
For example, the atomic number of carbon atom is 6. It means that one atom of carbon has
6 protons and 6 electrons.
11.8 Atomic Mass or Atomic Weight
The atomic mass or atomic weight of an atom is the sum of total number of protons and
neutrons. It is denoted by A. Since the mass of electrons is very less as compared to that
of protons and neutrons, the mass of electrons is neglected while calculating the atomic
mass.
Atomic mass (A) = No. of p+ + No. of n0
Similarly, No. of n0 = Atomic weight (A) – No. of p+
No. of n0 = Atomic weight (A) – Atomic number (Z)
∴ Atomic number (Z) = No. of p+
Above relation is used to find out the number of protons, neutrons and electrons from the
atomic number and atomic mass or atomic weight of an atom.
Worked out Numerical 1
The atomic number of a chlorine atom is 17 and its atomic weight is 35. Calculate the
number of protons, neutrons and electrons present in that atom.
Solution:
Given , Atomic no. (Z) = 17
Atomic weight (A) = 35
Now, Atomic no. (Z) = No. of p+ = No. of e– in a neutral atom
= 17 = 17
∴ The no. of protons (p+) = 17
The no. of electrons (e–) = 17
Again, Atomic weight (A) = No. of p+ + No. of n0
configuration /kənˌfɪɡəˈreɪʃn/ - an arrangement of the part of sth
154 Oasis School Science and Environment - 8 CHEMISTRY
or, 35 = 17 + No. of n0
or, No. of n0 = 35 – 17 = 18
∴ The number of neutrons (n0) = 18
11.9 Electronic Configuration and 2n² Rule
Electrons revolve around the nucleus of an atom in elliptical orbits or shells. In 1921 AD,
Bohr and Bury proposed a scheme to describe the arrangement of electrons in various
orbits or energy levels around the nucleus of an atom. The systematic distribution of
electrons in various orbits or energy levels around the nucleus of an atom is called
electronic configuration.
The shells or orbits are named as K, L, M, N, O, P and Q according to the distance from the
nucleus of an atom as shown in the given figure. Electrons are distributed systematically
in different shells or orbits. The number of electrons that can be accommodated by a
certain shell or orbit can be determined by 2n² rule. The rule that determines the maximum
number of electrons that can be accomodated in different shells of an atom is called 2n²
rule. In 2n² rule, 'n' represents the number of shells or orbits.
Distribution of electrons
Nucleus
Shells
Fig.11.4 Electronic distribution in different shells
The total number of electrons accomodated by the first four shells, i.e. K, L, M and N is
given below:
Shell or Orbit No. of Shell No. of electrons accomodated by the shell
K n = 1 2n² = 2 × 12² = 2
L n = 2 2n² = 2 × 22² = 8
M n = 3 2n² = 2 × 32² = 18
N n = 4 2n2² = 2 × 42² = 32
2n² rule is applicable to a few elements and shells K, L, M and N. But this rule is not
accommodate / ə ˈ k ɒ m ə d e ɪ t / - to provide enough space for sth
CHEMISTRY Oasis School Science and Environment - 8 155
applicable for the shells O, P and Q since the outermost shell cannot accomodate more
than 8 electrons. The maximum number of electrons that can be accomodated by the
shells O, P and Q is 32, 18 and 8 respectively. The energy level increases gradually from
the first shell to the last shell of an atom. So, electrons are filled from the first shell, i.e.
shell K to the higher shells, i.e. from lower energy level to the higher energy level.
The atomic number, symbol, atomic weight, number of protons, neutrons and electrons,
electronic configuration and valency of the first 20 elements are tabulated below:
Atomic Name of Symbol Number of Atomic Electronic Valency
Number Elements Weight Configuration
H P+ n0 e– (p++n0) K LMN 1
He 0
1. Hydrogen Li 1 01 1+0 = 1 1 × × × 1
2. Helium Be 2 22 2
3. Lithium B 3 43 2+2 = 4 2 × × × 3
4. Beryllium C 4 54 4
5. Boron N 5 65 3+4 = 7 2 1 × × 3
6. Carbon O 6 66 2
7. Nitrogen F 7 77 4+5 = 9 2 2 × × 1
8. Oxygen Ne 8 88 0
9. Fluorine Na 9 10 9 5+6 = 11 2 3 × × 1
10. Neon Mg 10 10 10 2
11. Sodium Al 11 12 11 6+6 = 12 2 4 × × 3
12. Magnesium Si 12 12 12 4
13. Aluminium P 13 14 13 7+7 = 14 2 5 × × 3
14. Silicon S 14 14 14 2
15. Phosphorus Cl 15 16 15 8+8 = 16 2 6 × × 1
16. Sulphur Ar 16 16 16 0
17. Chlorine K 17 18 17 9+10 = 19 2 7 × × 1
18. Argon Ca 18 22 18 2
19. Potassium 19 20 19 10+10 = 20 2 8 × ×
20. Calcium 20 20 20
11+12 = 23 2 8 1 ×
12+12=24 2 8 2 ×
13+14 = 27 2 8 3 ×
14+14 = 28 2 8 4 ×
15+16 = 31 2 8 5 ×
16+16 = 32 2 8 6 ×
17+18 = 35 2 8 7 ×
18+22 = 40 2 8 8 ×
19+20 = 39 2 8 8 1
20+20 = 40 2 8 8 2
156 Oasis School Science and Environment - 8 CHEMISTRY
The atomic structure and electronic configuration of first twenty elements is given below:
1. Hydrogen (H) e–
K
Shells K LMN
No. of e– 1××× 1p+
0n0
Hydrogen e–
K
2. Helium (He)
2p+
Shell K LMN 2n0
No. of e– 2×××
Helium
3. Lithium (Li) 3p+ K L
4n0
Shells K LMN Lithium
No. of e– 21××
4. Beryllium (Be) 4p+
5n0
Shells K LMN KL
No. of e– 22××
Beryllium
5. Boron (B) K LMN 5p+
23×× 6n0 K L
Shells Boron
No. of e–
6. Carbon (C)
Shells K LMN 6p+ KL
No. of e– 24×× 6n0
Carbon
7. Nitrogen (N)
Shells K LMN 7p+ KL
No. of e– 25×× 7n0
Nitrogen
CHEMISTRY Oasis School Science and Environment - 8 157
8. Oxygen (O)
Shells K LMN 8p+
8n0 K L
No. of e– 26×× Oxygen
9. Fluorine (F)
Shells K LMN 9p+ K L
10n0
No. of e– 27××
Fluorine
10. Neon (Ne) 10p+ KL
Shells K LMN 10n0
No. of e– 28××
Neon
11. Sodium (Na)
Shells K LMN 11p+ KLM
12n0
No. of e– 281×
Sodium
12. Magnesium (Mg)
Shells K LMN 12p+ KLM
12n0
No. of e– 282×
Magnesium
13. Aluminium (Al)
Shells K LMN 13p+ KLM
14n0
No. of e– 283×
Aluminium
14. Silicon (Si) 14p+ KLM
14n0
Shells K LMN
No. of e– 284×
Silicon
158 Oasis School Science and Environment - 8 CHEMISTRY
15. Phosphorus (P)
Shells K LMN 15p+
16n0
No. of e– 285× KLM
Phosphorus
16. Sulphur (S)
Shells K LMN 16p+ KLM
No. of e– 286× 16n0
Sulphur
17. Chlorine (Cl)
Shells K LMN 17p+ KLM
No. of e– 287× 18n0
18. Argon (Ar) Chlorine
Shells K LMN 18p+ KLM
No. of e– 288× 22n0
Argon
19. Potassium (K)
Shells K LMN 19p+ KLMN
No. of e– 2881 20n0
Potassium
20. Calcium (Ca)
Shells K LMN 20p+ KLMN
No. of e– 2882 20n0
CHEMISTRY Calcium 159
Oasis School Science and Environment - 8
11.10 Symbol
A symbol is the abbreviation of the full name of an element which is represented by one
or two English letters. It is used to make the study of elements easy and fast. If the symbol
of an element has one letter, it is written in capital letter. If it has two letters, the first letter
is written in capital and the second letter in small.
Some examples of elements which are represented by single letter are given below:
Atomic Elements Symbols Atomic Elements Symbols
number number
1 Hydrogen H 15 Phosphorus P
5 Boron B
6 Carbon C 16 Sulphur S
7 Nitrogen N
8 Oxygen O 23 Vanadium V
9 Fluorine F
53 Iodine I
92 Uranium U
There are some cases where the names of elements start with the same letter. For example,
hydrogen and helium begin with 'H'. Carbon, chlorine, chromium, cobalt, cadmium, and
calcium start with 'C'. In such conditions, we take the first letter and another significant
letter to symbolize these elements.
Atomic Elements Symbols Atomic Elements Symbols
number Helium He number Magnesium Mg
2 12
3 Lithium Li 25 Manganese Mn
17 Chlorine Cl 30 Zinc Zn
20 Calcium Ca 40 Zirconium Zr
24 Chromium Cr 22 Titanium Ti
27 Cobalt Co 73 Tantalum Ta
48 Cadmium Cd
There are certain elements whose names are written in English but symbols are used from
the Latin or German name.
English names Latin/German names Symbols
Iron Ferrum (Latin name) Fe
Copper Cuprum (Latin name) Cu
Gold Aurum (Latin name) Au
symbolize /ˈsɪmbəlaɪz/ - to be a symbol of sth CHEMISTRY
160 Oasis School Science and Environment - 8
Silver Argentum (Latin name) Ag
Mercury Hydrargyrum (Latin name) Hg
Sodium Natrium (Latin name) Na
Potassium Kalium (Latin name) K
Tungesten Wolfan (German name) W
11.11 Radicals
Radicals are charged atoms or group of atoms having a common charge which act as
a single unit during a chemical reaction. They have either positive charge or negative
charge. Radicals are charged particles. So, they are highly reactive and least stable. Hence,
they do not occur in free form and make different types of compounds. On the basis of
electric charge, radicals are of two types:
1. Electropositive radicals or basic radicals
The atoms or group of atoms which have positive charge in them are called
electropositive radicals or basic radicals. Some examples of electropositive radicals
with their valencies are given below:
Radicals having Radicals having Radicals having Radical having
valency 1 valency 2 valency 3 valency 4
(Monovalent) (Bivalent) (Trivalent) (Tetravalent)
Hydrogen (H+) Beryllium (Be++) Boron (B+++) Stannic (Sn++++)
Lithium (Li+) Magnesium (Mg++) Aluminium (Al+++) Plumbic (Pb++++)
Sodium (Na+) Calcium (Ca++) Ferric (Fe+++) Silicon (Si++++)
Potassium (K+) Strontium (Sr++) Auric (Au+++)
Rubedium (Rb+) Barium (Ba++) Chromium (Cr+++)
Caesium (Cs+) Cupric (Cu++) Manganic (Mn+++)
Cuprous (Cu+) Mercuric (Hg++)
Mercurous (Hg+) Stannous (Sn++)
Ammonium (NH4+) Zinc (Zn++)
Aurous (Au+) Nickel (Ni++)
Manganous (Mn++)
radical / ˈ r æ d ɪ k l / - a charged atom or group of atoms having a common charge
CHEMISTRY Oasis School Science and Environment - 8 161
2. Electronegative radicals or acidic radicals
The atom or group of atoms which have negative charge in them are called
electronegative radicals or acidic radicals. Some examples of electronegative radicals
are given below:
Radicals having valency Radicals having valency Radicals having valency 3
1 (Monovalent) 2 (Bivalent) (Trivalent)
_ _ _ _ _ _
Fluoride (F ) Oxide (O ) Nitride (N )
_ _ _ _ _ _
Chloride (Cl ) Sulphide (S ) Phosphate (PO4 )
_ _ _ _ _ _
Bromide (Br ) Sulphite (SO3 ) Phosphide (P )
Iodide (I –) _ _ _ _ _
_ Carbonate (CO3 ) Phosphite (PO3 )
Nitrate (NO3 ) _ _
_ Sulphate (SO4 )
Nitrite (NO2 ) _ _
_ Zincate (ZnO2 )
Cyanide (CN ) – –
_ Silicate (SiO3 )
Hydroxide (OH ) _ _
_ Peroxide (O2 )
Chlorate (ClO3 ) _ _
_ Dichromate (Cr2O7 )
Bisulphate (HSO4 ) _ _
_ Thiosulphate (S2O3 )
Bicarbonate (HCO3 )
_
Metaluminate (AlO2 )
11.12 Valence Shell, Valence Electrons and Valency
The outermost shell of an atom from where loss or gain of electrons takes place is called
valence shell and the total number of electrons present in the valence shell (outer shell)
are called valence electrons. For example, valence electrons in sodium, magnesium and
chlorine are 1, 2 and 7 respectively. Valence electrons determine the valency of an atom.
The combining capacity of an element or a radical with another element or radical to form a
compound or molecule is called valency. In the past, valency was defined as the total number
of hydrogen atoms which are combined with an element during chemical combination. For
example:
In HCl, the valency number of chlorine is 1.
valency /ˈveɪlənsi/ - the combining capacity of an element or a radical to form a molecule
162 Oasis School Science and Environment - 8 CHEMISTRY
In H2O, the valency number of oxygen is 2.
In NH3, the valency number of nitrogen is 3.
In CH4, the valency number of carbon is 4.
But all compounds do not have hydrogen atoms. So, this concept has been modified and
a new concept is put forth. According to the new concept, "The total number of electrons
lost, gained or shared by an atom during chemical combination is called valency." Valency
of sodium is 1 because it loses one electron and valency of oxygen is 2 because it gains
two electrons. The valency of chlorine is 1 as it gains one electron from other elements.
Similarly, the valency of carbon is 4 as it shares four electrons during chemical combination.
Examples,
1. Find out the valency of aluminium in AlCl3.
In AlCl3, three atoms of chlorine combine with one atom of aluminium. So, the
valency of aluminium is 3.
2. Find out the valency of hydrogen and phosphate in H3PO4.
In H3PO4, three atoms of hydrogen combine with one phosphate radical. So,
valency of phosphate is 3 and that of hydrogen is 1.
11.13 Duplet State and Octet State
Helium is the first member of inert gases. It has only two electrons and according to the
2n² rule these two electrons are present in the first shell (K-shell). So, the arrangement of
two electrons in the K-shell of an atom is called duplet state. Since helium atom has duplet
state, it does not take part in chemical reaction and remains in atomic form.
Except helium, the other five elements have eight electrons in their valence shell. It is also
stable electronic configuration. The state of having eight electrons in valence shell (last
shell) of an atom is called octet state. The presence of two electrons in helium (He) and
eight electrons in Ne, Ar, Kr, Xe, and Rn is the main cause of stability of these elements.
Hence, they have zero combining capacity (valency).
The elements which have more or less than eight electrons in their last shell are chemically
unstable and they always try to achieve this condition which is called octet rule. So, the
tendency of elements by which they try to maintain eight electrons in their valence shell
(last shell) either by transferring or sharing of electrons is called octet rule. Similarly, some
elements like H, Li, Be, etc. try to maintain two electrons in the shell K (last shell) either by
transferring or sharing of electrons which is called duplet rule.
CHEMISTRY Oasis School Science and Environment - 8 163
Reasonable fact-1
Neon atom can exist freely in nature.
Neon has eight electrons in its outermost shell. So, neon atom can exist freely in
nature.
Differences between Duplet and Octet
Duplet Octet
1. The state of an atom having two 1. The state of an atom having eight
electrons in its valence shell is called electrons in its valence shell is called
duplet state. octet state.
2. The state of helium atom is called 2. The state of neon atom is called octet
duplet state. state.
11.14 Molecular Formula
The molecular formula of a molecule is the symbolic representation of the molecule of an
element or a compound in molecular form. It represents the actual number of atoms of
different elements in a molecule. For example, the molecular formula of sodium chloride
is NaCl and that of water is H2O. It means that one molecule of sodium chloride (NaCl)
consists of one atom of sodium (Na) and one atom of chlorine (Cl). Similarly, one molecule
of water (H2O) consists of two atoms of hydrogen (2H) and one atom of oxygen. (O) In
case of inert gases, i.e. He, Ne, Ar, Kr, Xe and Rn, the single atom represents atom as well
as molecule because they are monoatomic molecules. Elements like hydrogen, nitrogen,
oxygen, chlorine, bromine and iodine have two atoms in their molecule, viz. H2, N2, O2,
Cl2, Br2 and I2 respectively. So, they are called diatomic molecules.
Differences between Symbol and Molecular formula
S.N. Symbol S.N. Molecular Formula
1. The symbol of an element is the 1. The molecular formula is the symbolic
abbreviation of the full name of representation of a molecule of an
that element. element or a compound.
2. It represents one atom of an 2. It represents one molecule of an
element. element or a compound.
Examples: H, Na, K, etc. Examples: H2, NaCl, H2SO4, etc.
We should follow the following steps to write the correct molecular formula of a
molecule.
1. Write the symbol of basic (positive) and acidic (negative) radicals side by side.
2. Write the valency of each radical on upper right corner of each.
3. Exchange the valency of these radicals. Take HCF if it is necessary.
4.a bbreCvioamtiobni /nəeb rri aː vdɪ iˈ ecɪaʃlns/ w i-t h e ax schhoartn fgoremd ovf aal wenocrdy,. etc.
164 Oasis School Science and Environment - 8 CHEMISTRY
Examples: 2. Magnesium chloride 3. Aluminium chloride
Mg Cl Al Cl
1. Sodium chloride 21 31
Na Cl
11
11 12 13
NaCl MgCl2 AlCl3
4. Carbon tetrachloride 5. Sodium oxide 6. Ammonium sulphate
C Cl Na O N H4 SO 4
41 12 12
14 21 21
CCl4 Na2O (NH4)2SO4
7. Calcium sulphate 8. Aluminium nitrate 9. Calcium bicarbonate
Ca SO 42 Al NO 3
22 31 Ca2 HCO 3
2 1
22 13 12
CaSO4 Al(NO3)3 Ca(HCO3)2
Information Obtained from Molecular Formula
1. Molecular formula represents one molecule of a substance.
2. It indicates total number of atoms of the same or different element/s in each molecule.
3. It indicates percentage composition of each element present in the compound.
4. The valency or combining capacity of each element can be found from the molecular
formula.
Example: In water molecule (H2O), the valency of hydrogen is 1 and that of
oxygen is 2.
5. We can calculate molecular weight from the molecular formula. For example,
The molecular weight of water (H2O) = 2 × H + 1 × O
= 2 ×1 + 1 × 16 = 18 amu.
11.15 Molecular Weight
The total weight of the molecule of a substance is called molecular weight. It is calculated
by adding the atomic weights of all the atoms present in a molecule. So, the sum of atomic
weights of the atoms present in a molecule is called molecular weight. One molecule of
hydrogen (H2) contains two atoms of hydrogen. So the molecular weight of hydrogen
molecule is 2 since atomic weight of hydrogen atom (H) is 1.
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The molecular weights of some common molecules are given below:
S.N. Molecules Molecular Weights
1. Sodium chloride (NaCl) NaCl = 1 × Na + 1 × Cl
= 1 × 23 + 1 × 35
2. Water (H2O) = 58 amu
H2O = 2 × H + 1 × O
= 2 × 1 + 1 × 16
3. Carbon diodixde (CO2) = 18 amu
CO2 = 1 × C + 2 × O
= 1 × 12 + 2 × 16
= 44 amu
4. Calcium carbonate (CaCO3) CaCO3 = 1 × Ca + 1 × C + 3 × O
= 1 × 40 + 1 × 12 + 3 × 16
= 100 amu
5. Magnesium chloride (MgCl2) MgCl2 = 1 × Mg + 2 × Cl
= 1 × 24 + 2 × 35
6. Calcium chloride (CaCl2) = 94 amu
CaCl2 = 1 × Ca + 2 × Cl
= 1 × 40 + 2 × 35
7. Ammonium sulphate = 110 amu
[NH4)2SO4] (NH4)2SO4 = 2 × N + 8 × H + 1 × S + 4 × O
= 2 × 14 + 8 × 1 + 1 × 32 + 4 × 16
= 132 amu
11.16 Periodic Table
Scientists have discovered 118 elements so far. Among them, 92 elements are found in
nature and remaining 26 elements are synthesized artificially in laboratories. It is difficult to
study all these elements one by one. So scientists have arranged these elements into various
categories like groups, periods, blocks, etc. on the basis of their similarities and differences.
a. Mendeleev's Periodic Table
Russian chemist Dmitri Ivanovich Mendeleev studied the physical and chemical
properties of 63 known elements and their compounds. After the study, he arranged
all the known elements on the basis of increasing atomic weights. As a result, he
periodic table /ˌpɪərɪˈɒdɪk ˈteɪbl/ - the table made by grouping of elements on the basis of characteristics
horizontal /hɒrɪˈzɒntl/ - going across and parallel to the ground
166 Oasis School Science and Environment - 8 CHEMISTRY
found that elements with similar properties occur at regular intervals. On the basis
of above observation, Mendeleev formulated a periodic law in 1869 AD. According
to Mendeleev's periodic law, "The physical and chemical properties of elements are
a periodic function of their atomic weights." Periodic function means, if elements are
arranged in order of increasing atomic weights, the properties of the elements go on
changing with atomic weights. But after a certain interval, they repeat the properties
of previous elements and fall one below another in the same vertical column.
With the help of above periodic law, Mendeleev arranged elements according to
increasing atomic weights. Hence, he found a table known as Mendeleev's periodic
table. In Mendeleev's periodic table, elements are classified into horizontal rows
called periods and vertical columns called groups.
A part of Mendeleev's periodic table
Group I Group II Group III Group IV Group V Group VI Group VII Group VIII
Period 1 H
Period 2 Li Be B C N OF
Period 3 Na Mg Al Si P S Cl
Period 4 K Ca 1* Ti V Cr Mn Fe Co
Cu Zn 2* 3* As Se Br Ni
Period 5 Rb Sr Y Zr Nb Mo 4* Ru Rh
Ag Cd In Sn Sb Te I Pd
Period 6 Cs Ba La Hf Ta W Re Os Ir
Au Hg Th Pb Bi Po At Pt
Merits of Mendeleev's periodic table
1. Mendeleev's periodic table was the first scientific and systematic study of all
known 63 elements.
2. In Mendeleev's periodic table, there were many gaps for undiscovered
elements. After their discovery, they got the proper place in the table.
3. With the help of Mendeleev's periodic table, properties of several elements
were studied correctly.
Demerits of Mendeleev's Periodic Table
1. Mendeleev could not arrange hydrogen properly because it showed the
properties of both alkali metals (IA) and halogens (VIIA).
2. Mendeleev could not arrange Lanthanides and Actinides.
3. In Mendeleev's periodic table, some elements do not obey the periodic law. In
some places, elements having more atomic weight were placed before those of
CHEMISTRY Oasis School Science and Environment - 8 167
less atomic weight. For example, 58.927Co was placed before 58.628Ni.
4. In Mendeleev's periodic table, more reactive alkali metals (Li, Na, K, etc.)
and less reactive coinage metals (Cu, Ag, Au) are placed together in the same
group.
5. Inert gases or noble gases are absent.
b. Modern Periodic Table
A group of chemists led by Henery Moseley studied the chemical and physical
properties of all known elements. They found that properties of elements depended
more correctly on atomic number rather than atomic weight. They said that atomic
number is the fundamental property of the atom. Afterward, they proposed a new
law which is called modern periodic law. Modern periodic law states that, "Physical
and chemical properties of the elements are a periodic function of their atomic
numbers." After the discovery of modern periodic law, elements were arranged on
the basis of increasing atomic numbers. As a result, they found that elements having
similar properties fall one after another in the same vertical column called group. The
elements having gradual change in the properties were in the horizontal row called
period. Hence, a more correct table than Mendeleev's table was obtained which is
known as modern periodic table. So, "The table which is obtained after arranging
elements on the basis of increasing atomic numbers is called modern periodic table."
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s-Block Modern Periodic Table 0/18
IA/1
p-Block
1
IIIA/13 IVA/14 VA/15 VIA/16 VIIA/17
IIA/2
CHEMISTRY 2
d-Block
3
IIIB/3 IVB/4 VB/5 VIB/6 VIIB/7 VIII/8 VIII/9 VIII/10 IB/11 IIB/12
4
5
6
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Lanthanide series
Actinide series
f-Block
Alkali Alkaline Transition Basic Metalloids Non Halogens Noble Lanthan- Actinides
metals earth metals metals metals gases ides
metals
Differences between Modern Periodic table and Mendeleev's Periodic Table
S.N. Modern Periodic Table S.N. Mendeleev's Periodic Table
1 It is based on increasing atomic 1 It is based on increasing atomic
numbers. weights.
2 In this table, there are eighteen 2 In this table, there are eight
vertical columns. vertical columns.
3 In this table, there is fixed 3 In this table, there is no fixed
position for hydrogen, noble position for hydrogen, noble
gases, lanthanides and actinides. gases, lanthanides and actinides.
Periods
In modern periodic table, there are seven horizontal rows, which are called periods.
In a period, elements having gradual change in properties are placed one after
another. According to the number of elements, periods of modern periodic table are
divided into four groups. They are very short, short, long and very long.
S.N. Period Number of Types of period
elements
1. First
2. Second 2 Very short period
3. Third
4. Fourth 8 Short period
8 Short period
18 Long period
5. Fifth 18 Long period
Very long period
6. Sixth 32 Very long but incomplete period
7. Seventh 26
Groups
In modern periodic table, there are a total of 18 vertical columns (IUPAC system)
which are called groups. In a group, elements having similar properties are placed
one below another.
In modern periodic table, representative elements are kept in groups IA, IIA, IIIA,
IVA, VA, VIA and VIIA. These elements are called representative elements or normal
elements as they have only one incomplete outer shell. Noble gas elements are kept
in 0 (zero) group or group VIIIA. Similarly, transitional elements are kept in groups
IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII. The group VIII is further divided into three
vertical columns.
transition element / t r æ n ˈ z ɪ ʃ n / ˈ e l ɪ m ə n t / - one of the elements in the centre of the modern periodic
table in the d-Block
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Differences between Periods and Groups
S.N. Periods S.N. Groups
1. The horizontal rows of periodic 1. The vertical columns of periodic
table are called periods. table are called groups.
2. In periods, atomic size of 2. In groups, atomic size of elements
elements decreases while increases while moving from top to
moving from left to right. bottom.
3. Valency of elements increases 3. Valency of elements remains the
first (upto group IV) and then same in a group.
decreases (upto group 0).
4. The elements of the same period 4. The elements of the same group
have different properties. have similar properties.
11.17 Chemical Reaction
When hydrogen gas (H2) burns in oxygen (O2), it forms water (H2O). Here, hydrogen
combines with oxygen and forms water. When calcium carbonate (CaCO3) is heated, it
decomposes or breaks down into calcium oxide (CaO) and carbon dioxide (CO2). Similarly,
when zinc (Zn) reacts with sulphuric acid (H2SO4), it displaces hydrogen from sulphuric
acid and forms zinc sulphate (ZnSO4) and hydrogen (H2) gas. So, the combination,
decomposition or displacement that occurs in the molecules of matter during a chemical
change is called chemical rection.
Examples:
1. When hydrogen gas burns in oxygen gas, it forms water, i.e.
Hydrogen + Oxygen burn Water
H2 + O2 burn H2O
2. When calcium carbonate is heated, it forms calcium oxide and carbon dioxide, i.e.
Calcium carbonate heat Calcium oxide + Carbon dioxide
CaCO3 ∆ CaO + CO2
3. When zinc reacts with sulphuric acid, it forms zinc sulphate and hydrogen gas, i.e.
Zinc + Sulphuric acid Zinc sulphate + Hydrogen
Zn + H2SO4 ZnSO4 + H2
A chemical reaction is expressed in word equation and chemical equation or formula
equation.
equation /ɪˈkweɪʒn/ - a statement showing that two amounts are equal
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11.18 Word Equation
The chemical reaction expressed by writing the full names of reactants and products is
called a word equation.
Examples: 1. Hydrogen + Oxygen Burn Water
2. Sodium + Chlorine Sodium chloride
Calcium oxide + Carbon dioxide
3. Calcium carbonate Heat Zinc chloride + Hydrogen
4. Zinc + Hydrochloric acid Contact
11.19 Chemical Equation
The chemical reaction expressed by writing symbols and molecular formulae of reactants
and products is called chemical equation. A chemical equation is more informative than
a word equation.
Examples1: . 2H2 + O2 ∆ 2H2O
heat 2NaCl
2. 2Na + Cl2 CaO + CO2
3. CaCO3 ∆
4. Zn + 2HCl ZnCl2 +H2
11.20 Reactants and Products
The chemical substances which take part in a chemical reaction are called reactants. The
chemical substances which are produced after chemical reaction are called products.
Reactants are written on the left side of the arrow whereas products are written on the
right side of the arrow while writing a chemical equation.
For example,
HCl + NaOH NaCl + H2O
Reactants Products
Differences between Reactants and Products
Reactants Products
1. The chemical substances which take 1. The chemical substances which are pro-
paret in a chemical reaction are called duced after chemical reaction are called
reactants. products.
2. Reactants are written on the left side 2. Products are written on the right side
of the arrow while writing a chemical of the arrow while writing a chemical
equation. equation.
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11.21 Unbalanced or Skeleton Chemical Equation
The chemical equation in which the total number of atoms of each element in reactants
and products are not equal is called unbalanced or skeleton chemical equation.
For example: H2 + O2 H2O
In above equation, the number of oxygen atoms in reactant and product sides is not
equal so it is called unbalanced chemical equation. Some more examples of unbalanced
chemical equations are as follows:
Mg + HCl MnO2 MgCl2 + H2
KOH + H2SO4 K2SO4 + H2O
H2O2 H2O + O2
HCl + Ca(OH)2 CaCl2 + H2O
11.22 Balanced Chemical Equation
The chemical equation written by balancing the total number of atoms of each element in
reactants and products is called balanced chemical equation. In this chemical equation,
the number of atoms of each element is equal in reactants and products. It gives more
information than the unbalanced chemical equation. Above unbalanced chemical
equations can be balanced as follows:
2H2 + O2 ∆ 2H2O
Mg + 2HCl MgCl2 + H2
2KOH + H2SO4 MnO2 K2SO4 + 2H2O
2H2O 2 2H2O + O2
2HCl + Ca(OH)2 CaCl2 + 2H2O
In the above chemical equations, the number of atoms of the same element in reactant and
product sides are equal. So, they are called balanced chemical equations.
Reasonable fact-3
Formula equation is more meaningful than a word equation.
Word equation represents the names of reactants and products in the form word
whereas formula equation represents the names of reactants and atoms and molecules
present in reactants and products. So, formula equation is more meaningful than a
world equation.
catalyst / ˈ k æ t ə l ɪ s t / - a chemical substance which increases or decreases the rate of chemical reaction
CHEMISTRY Oasis School Science and Environment - 8 173
11.23 Methods of Writing Balanced Chemical Equation
Following points should be remembered while balancing the chemical equation:
1. First of all, the chemical change is written correctly in the form of word equation.
For example: Hydrogen + Oxygen Water
2. The word equation is written correctly in the form of formula equation or chemical
equation.
For example: H2 + O2 H2O
3. The number of atoms of each element are balanced by using suitable coefficient
without changing the molecular formulae of reactants and products.
For example: 2H2 + O2 2H2O
4. The number of atoms in the biggest molecule should be balanced before balancing
the number of hydrogen and oxygen atoms.
This method of balancing chemical equation is called hit and trial method.
11.24 Some More Examples of Balanced Chemical Equation
1. Word equation : Sodium + Chlorine Sodium chloride
Unbalanced formula equation : Na + Cl2 NaCl
Balanced formula equation: 2Na + Cl2 2NaCl
2. Word equation : Potassium + Oxygen Potassium oxide
Unbalanced formula equation : K + O2 K2O
Balanced formula equation: 2K + O2 2KO
3. Word equation : Magnesium + Oxygen Magnesium oxide
Unbalanced formula equation : Mg + O2 MgO
Balanced formula equation: 2 Mg + O2 2MgO
4. Word equation : Zinc + Hydrochloric acid
Zinc chloride + Hydrogen
Unbalanced formula equation: Zn + HCl ZnCl2 + H2
Balanced formula equation: Zn + 2HCl ZnCl2 + H2
5. Word equation: Sulphuric acid + Sodium hydroxide Sodium sulphate + Water
Unbalanced formula equation: H2SO4+NaOH Na2SO4 + H2O
Balanced formula equation: H2SO4 + 2NaOH Na2SO4 + 2H2O
174 Oasis School Science and Environment - 8 CHEMISTRY
11.25 InformationObtainedfromaBalancedChemicalEquation
Following information can be obtained from a balanced chemical equation.
1. The names of reactants and products
2. The symbols and molecular formulae of reactants and products
3. The total number of atoms or molecules of reactants and products
4. The ratio of molecular weight of reactant and product molecules
5. The type of chemical reaction
11.26 Limitation of a Balanced Chemical Equation
A balanced chemical equation cannot provide information about
1. The physical state of reactants and products
2. Concentration of reactants
3. Conditions required for the reaction like heat, light, pressure, catalyst, etc.
4. The duration of the chemical reaction
11.27 Modification of Chemical Equation
To make the chemical reaction more informative following modifications are done.
1. The physical state of reactants and products are denoted by 's' for solid, 'l' for liquid,
'g' for gas and 'aq' for aqueous solution.
2. Concentration of reactants are denoted by 'dil.' for dilute and 'conc.' for concentrated solution.
3. The conditions like temperature, pressure, light, catalyst, etc. are written above or
below the arrow.
4. A double-way arrow ( ) is used for reversible reaction and a single way arrow (→)
is used for the irreversible reaction.
For example,
2Na(s) + 2H2O (l) 2NaOH (aq) + H2↑
heat / pressure
N2(g) + 3H2(g) catalyst / promoter 2NH3 ↑
2HgO (s) heat 2Hg (l) + O2 ↑
11.28 Reversible Reaction
The chemical reaction in which the products can recombine to give back the reactants is
called reversible reaction. For example,
H2 + I2 heat 2HI
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When hydrogen (H2) and iodine (I2) are heated, hydrogen iodide (HI) is formed. When
hydrogen iodide is heated in a closed vessel, it also forms hydrogen and iodine. Therefore,
the given reaction is a reversible reaction. Reversible reactions are written by giving a
double-way arrow between reactants and products as follows:
H2 + I2 2HI
N2 + 3H2 2NH3
2H2 + O2 2H2O
11.29 Irreversible Reaction
The chemical reaction in which the products cannot recombine to give back reactants is
called irreversible reaction. A single way arrow is used to denote an irreversible reaction.
Examples:
CaCO3 ∆ CaO + CO2↑
2Na + 2H2O 2NaOH + H2↑
2KClO3 ∆ 2KCl + 3O2↑
Activity 1
• Bring a chart paper and draw the atomic models of the first twenty elements.
• Put suitable colours in the pictures and submit to your science teacher.
176 Oasis School Science and Environment - 8 CHEMISTRY
SUMMARy
• Anything which has mass and volume is called matter.
• A compound is a substance made up of two or more elements (atoms) in a fixed
proportion by weight.
• Elements are the pure substances made up of only one type of atoms.
• An atom is the smallest particle of an element which can take part in a chemical reaction.
• A molecule is the smallest particle of an element or a compound which can exist freely.
• Electrons are negatively charged sub-atomic particles that revolve around the
nucleus in elliptical orbits called shells.
• Atomic number of an atom is defined as the number of protons present in the
nucleus of that atom.
• The systematic distribution of electrons in various orbits or energy levels around
the nucleus of an atom is called electronic configuration.
• The rule that determines the maximum number of electrons that can be accomodated
in different shells of an atom is called 2n² rule.
• A symbol is the abbreviation of the full name of an element which is represented by
one or two English letters.
• Radicals are charged atoms or group of atoms having a common charge which act
as a single unit during a chemical reaction.
• The combining capacity of an element or a radical with another element or radical
to form a compound or molecule is called valency.
• The arrangement of two electrons in the K-shell of an atom is called duplet state.
• The state of having eight electrons in valence shell (last shell) of an atom is called
octet state.
• The molecular formula of a molecule is the symbolic representation of the molecule
of an element or a compound in molecular form.
• The sum of atomic weights of the atoms present in a molecule is called molecular weight.
• According to Mendeleev's periodic law, "The physical and chemical properties of
elements are a periodic function of their atomic weights."
• Modern periodic law states that, "Physical and chemical properties of the elements
are a periodic function of their atomic numbers."
• The combination, decomposition or displacement that occurs in the molecules of
matter during a chemical change is called chemical reaction.
• The chemical substances which take part in a chemical reaction are called reactants.
The chemical substances which are produced after chemical reaction are called
products.
• The chemical equation written by balancing the total number of atoms of each
element in reactants and products is called balanced chemical equation.
• The chemical reaction in which the products can recombine to give back the
reactants is called reversible reaction.
CHEMISTRY Oasis School Science and Environment - 8 177
Exercise
1. Choose the best answer from the given alternatives.
a. The smallest particle of an element is called ............................. .
i. molecule ii. matter iii. atom iv. compound
b. The sub-atomic particles that revolve around the nucleus are called ............... .
i. protons ii. neutrons iii. electrons iv. orbits
c. The number of electrons present in 1 coulomb charge is ......................... .
i. 6.25 × 1018 ii. 62.5 × 1018 iii. 65.2 × 1018 iv. 625 × 1018
d. The maximum number of electrons accomodated by shell L is ..................... .
i. 8 ii. 18 iii. 2 iv. 6
e. The molecular formula of Aluminium oxide is .......................... .
i. Al3O2 ii. Al2O3 iii. AlO3 iv. Al2O2
f. The molecular weight of Calcium carbonate (CaCO3) is ......................... .
i. 100 amu ii. 1000 amu iii. 80 amu iv. 50 amu
2. Tick (√) the correct statement and cross (×) the incorrect one.
a. Scientists have discovered 118 elements so far.
b. All molecules are made of similar type of atoms.
c. Neutrons have positive charge.
d. The atomic weight of potassium is 40.
e. The valency of sodium (Na) is 1.
f. Word equation is more informative than formula equation.
3. Fill in the blanks with appropriate words.
a. The smallest particle of a compound is called ........................... .
b. The mass of one proton is equal to that of one ........................... atom.
c. The sum of number of protons and neutrons is called ......................... .
d. Electrons revolve around the nucleus in elliptical .......................... .
e. The table in which elements are arranged on the basis of properties is called
...................... .
f. A chemical reaction is expressed in the form of ...................... equation and
...................... equation.
g. Reactants are written on the ...................... side of the arrow.
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4. Answer the following questions.
a. Define atom and molecule.
b. What are elements? Give any three examples.
c. What are compounds? Give any three examples.
d. What are sub-atomic particles? Write down their position in an atom.
e. What is meant by the atomic mass of an atom? Write down the atomic mass of
oxygen, sodium, aluminium and argon.
f. Define atomic number and atomic weight.
g. What is 2n² rule? Calculate the maximum number of electrons found in shell
N by using this rule.
h. What is valency? Write down the valency of oxygen, magnesium and calcium.
i. What are valence electrons? Write with an example.
j. What is meant by duplet and octet state? Describe with an example of each.
k. What is molecular formula? Write down the method of writing molecular
formula.
l. What is molecular weight? How does it differ from atomic weight?
m. What is meant by periodic table? State Mendeleev's and modern periodic rule.
n. What is chemical reaction? Give any two examples.
o. What are reactants and products? Write with an example.
p. Define word equation and chemical equation with any two examples of each.
q. What is a balanced chemical equation? Give any three pieces of information
that can be obtained from a balanced chemical equation.
5. Differentiate between:
a. Atom and Molecule
b. Element and Compound
c. Proton and Electron
d. Mendeleev's periodic table and Modern periodic table
CHEMISTRY Oasis School Science and Environment - 8 179
e. Group and Period
f. Reactant and Product
g. Word equation and Chemical equation
6. Give reason:
a. Atoms are electrically neutral.
b. The valency of chlorine is 1.
c. Chemical equation is more informative than word equation.
7. Describe the structure of an atom with a neat diagram.
8. Draw the atomic structure of nitrogen, aluminium, chlorine and calcium.
9. Write down the molecular formula of each of the given compounds.
a. Sodium chloride
b. Calcium oxide
c. Magnesium chloride
d. Calcium carbonate
e. Water
f. Potassium sulphate
g. Sodium carbonate
10. Calculate the molecular weight of each of the given compounds.
a. NaCl b. CaCO3 c. Na2SO4
d. CaCl2 e. NH4OH f. MgSO4
11. Complete the given word equations and express them in the form of balanced
formula equations.
a. Hydrogen + Oxygen ∆ ..................
b. Calcium carbonate ∆ .................. + ..................
c. Water electrolysis .................. + ..................
d. Zinc + .................. Zinc sulphate + ..................
e. Sodium + ................ Sodium hydroxide + ..................
180 Oasis School Science and Environment - 8 CHEMISTRY
12UNIt Estimated teaching periods : Th Pr
3 2
Paper Chromatography
MIxtUre
Objectives
After completing the study of this unit, students will be able to:
• explain and demonstrate distillation and fractional distillation.
• explain and demonstrate chromatography.
Course of Study
• Distillation – Introduction
• Types of distillation
• Simple distillation
• Fractional distillation
• Chromatography and its principle
• Application of chromatography
Points to be Focused/Questions to be Discussed
• What is distillation?
• What types of mixtures are separated by distillation?
• What is fractional distillation?
• What is chromatography?
• What is the principle of chromatography?
• What is the application of chromatography?
CHEMISTRY Oasis School Science and Environment - 8 181
12.1 Introduction
Most of the substances around us are found in the form of mixtures. A mixture contains
more than one substance. The components of a mixture are mixed in any proportion and
do not undergo any chemical change. When two or more substances are brought together
in any proportion and do not undergo any chemical change, the resulting mass is called
mixture. Air, tap water, soil, milk, tea, coffee, etc. are some examples of mixture.
Mixture can be classified into two types, viz. homogeneous mixture and heterogeneous
mixture. The state of a mixture may be solid, liquid or gas. Similarly, a mixture may
contain different solids, solid and liquid, or liquid and gas. But the mixture of solid and
gas is not found in nature.
The components of a mixture can be separated by simple physical methods. The different
components of a mixture have different physical properties like size, solubility, density,
state, etc. These properties are utilized to separate them from a mixture. Various methods
are used to separate the components of different mixtures. In this unit, we will discuss
only two methods of separating the components of mixtures, viz. (i) distillation and (ii)
chromatography.
Differences between Homogeneous mixture and Heterogeneous mixture
Homogeneous mixture Hetergeneous mixture
1. It is a mixture where the components 1. It is a mixture where the components
that make up the mixture are uniformly that make up the mixture are not
distributed throughout the mixture. uniformly distributed throughout the
mixture.
2. In this mixture, all the components of 2. In this mixture, all the components of
mixture cannot be seen by our eyes. mixture can be seen by our eyes.
12.2 Distillation
Distillation is a method to separate a volatile liquid from a non-volatile solid or the
mixture of two or more liquids having different boiling points. In this process, a liquid
is changed into vapour by heating and on subsequent condensation of the vapour, turns
back into liquid. So, distillation is the method that is used to separate liquids from solids
or from other liquids through vaporization (evaporation) and condensation. Mixtures like
salt and water, iodine and alcohol, potassium chloride and water, alcohol and water, etc.
can be separated by distillation. This process is also used to purify a liquid.
homogeneous /ˌhɒməʊˈdʒiːnɪəs/ - consisting of things that are all of the same type
heterogeneous /ˌhetərəʊˈdʒiːnɪəs/ - consisting of things that are of different types
182 Oasis School Science and Environment - 8 CHEMISTRY
Thermometer Hot water out
Stand Liebig's condenser Cold water in
Round bottom Conical flask
flask (reciever)
Wire gauze
Mixture
Bunsen burner
Fig 12.1 Distillation
Distillation usually involves boiling a liquid and condensing the vapour that forms. When
liquid boils it changes into vapour. The vapour is collected and condensed to form liquid
which is collected into another container.
The apparatus used in distillation is called still which consists of a boiler, a condenser
(Liebig's condenser) and a receiver. The mixture to be vapourized is heated in the boiler.
Whichever substance in the mixture boils at the lowest temperature will be the first to
turn into vapour. The vapour is passed through the condenser where it cools and turns
into liquid again. The distilled liquid, called the distillate, is collected in the receiver.
12.3 Types of Distillation
i. Simple distillation
ii. Fractional distillation
i. Simple distillation
Simple distillation is used to obtain a pure S. N. Substances Boiling point
substance that dissolves another substance. 1. Water 100 0C
The solution is boiled and the vapour that 2. Alcohol 78 0C
condenses consists of the pure solvent. This 3. Mercury 357 0C
method is used to obtain distilled water. 4. Chloroform 61 0C
Similarly, the mixture of salt and water can 5. Sea water 103 0C
be separated by this process.
Simple distillation is also used to separate a mixture of two miscible liquids having
different boiling points of a wide range. When the boiling point of the liquid having
condenser /kənˈdensə(r)/ - a device that cools gas in order to change it into a liquid
CHEMISTRY Oasis School Science and Environment - 8 183
a lower boiling point is reached, its vapour will be formed. The vapour is passed
through Liebig's condenser and the liquid formed after condensation is collected
into a receiver. The thermometer will show the same constant reading till the whole
of the liquid has been evaporated. Now, it is the turn of the second liquid of the
mixture. When boiling point of the second liquid is reached by increasing the
temperature, it also changes into vapour. The vapour passes through the condenser
and changes into liquid which is collected into a second receiver. In this way, liquids
having different boiling point (e.g. mixture of alcohol and water) can be separated
by simple distillation.
Activity 1
Objective: To separate the mixture of common salt and water
Materials required: Mixture of salt and water, round bottom flask, Bunsen burner,
wire gauze, stands, tripod stand, thermometer, condenser, conical flask, match box,
cork and delivery tube
Procedure
• Take a round bottom flask and keep the solution of salt and water.
• Keep the flask on the tripod stand and support it with another stand.
• Connect the flask with a condenser as shown in the figure.
Stand Thermometer
Water bath Water out
Mixture of
Liebig's
salt and water condenser
Water in Beaker
Burner (reciever)
Tripod stand
Fig.12.2
• Arrange the flow of cold water in the condenser.
• Connect a thermometer on the mouth of the round bottom flask.
• Now, keep a receiver on the free end of the condenser.
• Boil the solution of salt and water with burner till all the water evaporates.
Observation
When water boils, it evaporates. The vapour condenses into liquid while passing
through the condenser. The liquid thus formed collects in the receiver and the salt
remains in the round bottom flask.
Conclusion
In this way, the mixture of salt and water can be separated by distillation.
184 Oasis School Science and Environment - 8 CHEMISTRY
ii. Fractional distillation
Fractional distillation is used to separate the mixture of two or more liquids of close
boiling range. The liquid mixture is heated to the temperature at which one of the
liquids boils. The resulting vapour is cooled until it condenses to give pure liquid.
This process is repeated until all liquids in the mixture have been separated.
In fractional distillation, a fractionating column is used in addition to the materials
for simple distillation. A mixture of alcohol and water is distilled to separate them.
The boiling point of alcohol is 78. 5 0C. When the column reaches this temperature
on heating, the alcohol continues to evaporate, but the water which turns back to
a liquid at temperature below 100 0C condenses and falls down the column back
into the flask. Thus, alcohol is separated from water. When all the alcohol changes
into vapour, the temperature rises and water boils when temperature reaches
100 0C. Water vapour changes into liquid when it passes through the condenser
which is collected into a separate receiver.
Stand Thermometer
Water out
Liebig's condenser
Fractional column
Mixture of Stand Water in
alcohol and
Round bottom flask water Reciever
Burner
Fig 12.3 Fractional distillation
Fractional distillation is widely used in industries. A spirit distillery distills alcoholic
liquors. An oil refinery distills crude oil, separating it into petrol, paraffin, petroleum
jelly, fuel oil, lubricating oil, etc. Similarly, liquid air is distilled to separate nitrogen
and oxygen gas.
Differences between Simple distillation process and Fractional distillation
Simple distillation Fractional distillation
1. It is used to separate a mixture 1. It is used to separate the mixture of two
of two miscible liquids having or more liquids of close boiling range.
different boiling points.
2. In simple distillation plant, there is 2. In fractional distillation plant, a
no fractional column. fractional column is used.
lubricate /ˈluːbrɪkeɪt/ - to put a lubricant on sth such as the parts of a machine, to help them move smoothly
CHEMISTRY Oasis School Science and Environment - 8 185
12.4 Chromatography
Chromatography is a modern technique for separation of substances of a mixture based
on the difference in adsorption of different constituents on the surface of a solid. It works
on the principle of differential adsorption of substances on the surface of the adsorbent.
Different substances move with different speed when they pass through a fixed phase. The
fixed phase may be a filter paper, silica gel, chalk powder, alumina or cellulose powder.
The word chromatography has been derived from two Greek words 'kroma' and 'graphy'.
Here, kroma means colours and graphy means to write. So, different colours can be separated
from their mixture by means of chromatography. However, colourless substances can also
be separated by this method. Chromatography was invented by the Russian botanist Tswett
in 1906 AD to separate the coloured pigments of plants. Nowadays, chromatography is
widely used to separate different types of mixture of solids, liquids and gases and even
when small quantities of substances are available.
Reasonable fact-1
The mixture of varius colours can be separated by chromatography.
Different colours move with different speed when passed through the same medium.
So, the mixture of various colours can be separated by chromatography.
In chromatography, we have to dissolve the mixture in a moving phase which could be
a liquid or a gas and then to pass over a fixed phase. Depending on the nature of the
moving and fixed phases, chromatography is of three types.
i. Paper chromatography
ii. Column chromatography
iii. Gas chromatography
In this unit, we will discuss paper chromatography and column chromatography.
i. Paper chromatography
Paper chromatography is based
on the fact that porous paper
adsorbs different substances on
different extremes. The mixture
to be separated is first dissolved
in a liquid. A small drop of the
solution is then placed about 1 cm
from the edge of the strip of filter
paper. After the solution dries, the Fig 12.4 Paper chromatography
strip of paper is hung vertically
in a glass vessel. In this position,
the lower edge of the paper strip makes contact with the layer of solvent in the
vessel. The solvent rises in the paper and carries various substances in the mixture
186 Oasis School Science and Environment - 8 CHEMISTRY
to different heights on the paper strip, thereby separating the components of the
mixture. The mixture of various colours can be separated by paper chromatography.
Activity 2
Objective: To separate various colour from the mixture of red, blue and green inks by
paper chromatography
Materials required: Mixture of red, blue and green inks, filter paper, dropper, beaker,
petridish
Procedure
• Take a beaker and put some water into it.
• Take a petridish and prepare a solution of red, blue and green ink.
• Take a filter paper and make a small hole at its centre and insert a roll of filter
paper into the hole.
pFaiplteerr Mixture of ink
Beaker Roll of
filter paper
Water
Fig 12.5 Separating mixture of red,
blue and green inks
• Now, put one drop of the mixture of inks near the centre of the filter paper and
let it dry for a while.
• Now, keep the roll of the filter paper on the beaker so that the lower end of the
roll of the filter paper touches the water in the beaker.
• Leave the beaker for 2-3 hours and then observe it.
Observation
After 2-3 hours, patterns of different colours can be seen on the filter paper.
Conclusion
In this way, various colours can be separated from their mixture by paper
chromatography.
CHEMISTRY Oasis School Science and Environment - 8 187
Activity 3
Objective: To separate colours from the mixture of blue and red ink
Materials required: Beaker, water, filter paper, scissors, glass rod, tape, water, mixture
of blue and red ink
Procedure
• Take beaker and put 3-4 drops of blue and red ink in the beaker. Add 5 ml of
water in the beaker.
• Take a filter paper and cut a rectangular strip of the paper with scissors.
• Now, adjust the strip of the filter paper in the beaker with the help of glass rod and
a tape in such a way that the lower end of the filter paper touches the mixture in
the beaker. The strip should not touch the wall of the beaker.
• Leave the beaker for 1-2 hours and then observe it.
Glass rod
Beaker
Filter paper
Mixture of red
and blue ink
Fig 12.6
Observation
After 1-2 hours, separate patterns of different colours can be seen in the strip of the
filter paper.
Conclusion
In this way, various colours present in the mixture of red and blue ink can be separated
by paper chromatography.
i. Column chromatography
In column chromatography, the mixture
of two or more dissolved substances is
passed through an adsorbent like silica
gel, alumina, chalk powder, cellulose
powder, etc. Column chromatography is
also called adsorption chromatography
as we use an adsorbent in this method.
Adsorption chromatography is based
on the principle that an adsorbent can Fig. 12.7 Column/adsorption chromatography
alumina /əˈluːmɪnə/ - a white substance found in rocks, clay
adsorbent /ədˈzɔːbənt/ - a solid substance that adsorbs gases or liquids
188 Oasis School Science and Environment - 8 CHEMISTRY
adsorb different substances to different extents.
In column chromatography, the mixture of two or more dissolved substances to be
separated is allowed to pass through the vertical glass tube filled with an adsorbent
like silica gel, chalk powder, alumina, etc. When the mixture passes down through
the adsorbent, various colours are separated in the form of bands. In this way, various
colours or dissolved substances can be separated by column chromatography.
Activity 4
Objective: To separate various colours form the mixture of red, green, blue and
black ink by column chromatography
Materials required: Glass tube, chalk powder, silica gel, mixture of red, green, blue
and black ink, beakers, bunsen burner, match box, stand
Procedure
• Take a 15 cm long glass tube and heat its one Stand Mixture of ink
Solvent
end to make the lumen narrow.
• Fill the two-third part of the glass tube with Glass tube
chalk powder or silica gel. Adsorbent
(silica gel)
• Adjust the glass tube in vertical position with
the help of a stand. Beaker
• Pour the mixture of ink into the glass tube. Solvent
• Collect the ink of different colours one by one Fig 12.8
into separate beakers.
Conclusion
In this way, various colours can be separated from the mixture of ink by column
chromatography.
12.5 Applications of Chromatography
1. Chromatography is used to separate various colours from their mixture.
2. It is used to separate and identify the medicines mixed in blood and urine.
3. It is used to separate the components of natural and artificial colours.
CHEMISTRY Oasis School Science and Environment - 8 189
Reasonable fact-2
Chromatography is not suitable when two substances move with the same speed in
a medium.
Chromatography is not suitable when two substances move with the same speed in a
medium. This is because chromatography is used for separating different substances
that travel at different speeds when allowed to pass through the same medium.
Similarly, if two substances which have same speed, they are going to travel together
and they won't separate as chromatography separates substances having different
rates or speeds for travelling.
SUMMARy
• When two or more substances are brought together in any proportion and do
not undergo any chemical change, the resulting mass is called mixture.
• Distillation is the method that is used to separate liquids from solids or from
other liquids through vaporization (evaporation) and condensation.
• Simple distillation is used to obtain a pure substance that dissolves another
substance.
• Fractional distillation is used to separate the mixture of two or more liquids of
close boiling range.
• Chromatography is a modern technique for separation of substances of a
mixture based on the difference in adsorption of different constituents on the
surface of a solid.
• Paper chromatography is based on the fact that porous paper adsorbs different
substances on different extremes.
• Adsorption chromatography is based on the principle that an adsorbent can
adsorb different substances to different extents.
• Chromatography is widely used to separate different types of mixture of solids,
liquids and gases and even when small quantities of substances are available.
190 Oasis School Science and Environment - 8 CHEMISTRY
Exercise
1. Choose the best answer from the given alternatives.
a. When two or more substances are brought together in any proportion, the
resulting mass is called .........................
i. solution ii. mixture iii. element iv. compound
b. The mixture of salt and water can be separated by .........................
i. sedimentation ii. chromatography
iii. distillation iv. condensation
c. The boiling point of alcohol is .........................
i. 78.5 0C ii. 100 0C iii. 58.7 0C iv. 87.50C
d. The mixture of red ink and blue ink can be separated by ......................... .
i. crystallization ii. filtration iii. distillation iv. chromatography
2. Tick (√) the correct statement and cross (×) the incorrect one.
a. The components of a mixture do not undergo any chemical change.
b. The mixture of solid and gas is found in nature.
c. In distillation, evaporation is followed by condensation.
d. The mixture of alcohol and water can be separated by paper
chromatography.
e. Silica gel is an example of an adsorbent.
3. Fill in the blanks with appropriate words.
a. The mixture of ......................... and gas is not found in nature.
b. ......................... is used to purify liquids.
c. The boiling point of mercury is ......................... .
d. Fractional ......................... is widely used in industries.
e. Adsorbent is used in ......................... .
4. Answer the following questions.
a. What is a mixture? Give any two examples.
b. What is distillation? What type of mixtures can be separated by this method?
c. What is fractional distillation?
d. Write any two uses of fractional distillation.
CHEMISTRY Oasis School Science and Environment - 8 191
e. What is chromatography? Write down the principle of chromatography.
f. What is meant by paper chromatography and column chromatography?
g. Which method is used to separate various colours from their mixture?
h. Which method is used to get pure water from sea water?
5. Identify the method of separating the components of the following mixtures.
a. Salt and water
b. Water and alcohol
c. Red and green ink
d. Medicines mixed in urine
6. Differentiate between:
a. Distillation and Chromatography
b. Simple distillation and Fractional distillation
c. Paper chromatography and Column chromatography
7. Describe an experiment to separate the mixture of salt and water with a neat and
labelled figure.
8. Describe an experiment to separate various colours from the mixture of blue and
black ink with a neat and labelled figure.
9. "Chromatography is not suitable when two substances move with the same
speed in a medium." Justify this statement.
10. The liquids having the same boiling point cannot be separated by distillation.
Justify this statement.
192 Oasis School Science and Environment - 8 CHEMISTRY
13UNIt Estimated teaching periods : Th Pr
7 1
METAL AND NON-METAL
Objectives
After completing the study of this unit, students will be able to:
• define metals, non-metals and metalloids with examples.
• state the position of metals, non-metals and metalloids in the periodic table.
• introduce and explain the physical properties of some useful metals,
non-metals and metalloids (gold, silver, copper, iron, aluminium,
silicon and sulphur) with their uses.
Course of Study
• Introduction to metals, non-metals and metalloids
• Position of metals, non-metals and metalloids in the periodic table
• Some useful metals, non-metals and metalloids (gold, silver, copper, iron,
aluminium, silicon and sulphur), their physical properties and uses
Points to be Focused/Questions to be Discussed
• What are metals? Give examples.
• What are non-metals? Give examples.
• What are metalloids? Give examples.
• What is the position of metals, non-metals and metalloids in the
periodic table?
• What are the physical properties of gold, silver, copper, iron,
aluminium, silicon and sulphur?
• What are the uses of gold, silver, copper, iron, aluminium, silicon
and sulphur?
CHEMISTRY Oasis School Science and Environment - 8 193
13.1 Introduction
A variety of substances are found in our surroundings. Among them, some are pure while
others are impure. The pure substances are called elements. Scientists have discovered
118 elements so far. On the basis of their properties, elements are divided into three types.
They are metals, non-metals and metalloids. Most of the elements are hard, malleable,
ductile and good conductors of heat and electricity. These elements are called metals. For
example, copper, silver, gold, aluminium, iron, etc. Some elements are soft, non-malleable,
non-ductile and bad conductors of heat and electricity. These elements are called non-
metals. For example, carbon, sulphur, chlorine, oxygen, nitrogen, iodine, phosphorus, etc.
However, some other elements show the properties of both metals and non-metals. These
elements are called metalloids. For example, silicon, germanium, arsenic, etc.
13.2 Metals
Metals are the solid substances which are malleable, ductile and good conductors of
heat and electricity. They are lustrous substances and produce tinkling sound on hitting.
Copper, iron, aluminium, silver, gold, etc. are some examples of metals. Most of the metals
exist in solid state at the room temperature but mercury exists in liquid state. Metals are
widely used for making various objects of our daily use. Metals like iron, aluminium
and copper are used for making cooking utensils. Gold and silver are used for making
jewellery. Copper and aluminium are also used for making electric wires. Iron is also used
in construction works, for making vehicles, weapons, etc.
Fig.13.1 Metals are used for making different things
13.3 Non-metals
Non-metals are generally soft, non-malleable and non-ductile substances that can be found
in solid, liquid or gaseous state. Non-metals are bad conductors of heat and electricity
except graphite. Non-metals are also widely used in our day to day activities. They are
used to make medicines, utensils, containers, etc. They are also used in construction works.
Carbon, sulphur, phosphorus, iodine, chlorine, etc. are some examples of non-metals.
13.4 Position of Metals, Non-metals and Metalloids in the
Modern Periodic Table
In the modern periodic table, metals are placed in the left side except hydrogen. Metals are
placed in groups IA, II A, III A and groups IB to VIII. The metals of groups IA, II A and III
malleable /ˈmælɪəbl/ - that can be hit or pressed into shapes easily without breaking or cracking
ductile /ˈdʌktaɪl/ - that can be made into a thin wire
194 Oasis School Science and Environment - 8 CHEMISTRY
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