SEE class 10 classification of elements Notes by RIYa

Class10

Unit-7

Classification
of Elements

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Periodic table

The periodic table is a chart or table of the elements arranged into columns
and rows according to their physical and chemical properties. In short, it can be
said that the periodic table is made up of rows of elements and columns.

Uses of periodic table

1. It gives a clear and distinct classification of almost all elements.
2. It helps for the prediction of new elements.
3. It gives the correct position to elements.
4. It is possible to describe simple properties of elements from the periodic table.
5. It displays the atomic numbers, atomic weights, symbols and names of the elements along

with information about the structure of their atoms.

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Mendeleev's periodic law

Mendeleev's periodic law states that "the physical and the chemical properties of elements are the periodic
functions of their atomic mass". The periodic function means that the properties of elements go on changing
with atomic mass but are repeated after regular intervals. Mendeleev's periodic table is defined as the table or
chart of the elements arranged into rows and columns on the basis of increasing atomic masses or
Mendeleev's periodic law. In Mendeleev's periodic table the elements are arranged in vertical columns, called
groups (families) and horizontal rows called periods (series). Mendeleev arranged 63 known elements. Name
given by Mendeleev : 1* Eka - boron, 2* Eka - aluminium, 3* Eka - silicon, 4* Eka – manganese . Dmitri Ivanovich Mendeleev, a
Russian chemist was the first person to introduce to the concept of the periodic table.

Group Group Group Group Group Group Group Group
I II III IV V VI VII VIII

Period 1 H
Period 2
Period 3 Li Be B C N OF
Period 4 P S Cl
Na Mg Al Si V Cr Mn
Period 5 3* As Se
K Ca 1* Ti Nb Mo 4* Fe Co
Period 6 Sn Sb Te Br Ni
Cu Zn 2* Ta W Re
Pb Bi Po Ru Rh
Rb Sr Y Zr I Pd
Os Ir
Ag Cd In At P t

C s B a La Hf

Au Hg Th

Features of Mendeleev's periodic table

The original form of Mendeleev's periodic table contained six periods and eight groups. However, the
modified form of Mendeleev's periodic table had seven periods and nine groups.

1. Mendeleev arranged all the known 63 elements in increasing order of their atomic masses. The
elements with similar properties came under the same group after a regular interval.

2. Groups: The vertical columns of elements are groups. The groups were named as I, II, III, IV, V, VI, VII and
VIII. The groups I - VII were further divided into subgroups

A and B. The groups VIII had a set of 3 elements. In the modified form of the periodic table, the ninth
group is named as zero (0) group which contains noble gases.

3. Period: The horizontal rows of elements are called periods. In order to accommodate
more elements, the periods 4, 5 and 6 are divided into two halves.

Merits of Mendeleev's periodic table

1. Mendeleev's periodic table helps in the systematic and scientific study of the elements.

2. He classified all the known 63 elements at that time.

3. He left some gaps for the undiscovered elements and predicted the properties of such
unknown elements.

4. He found mistakes in the atomic masses of certain elements like beryllium, gold, platinum, uranium, etc.
during the construction of the periodic table e. g. atomic mass of beryllium. Mendeleev studied atomic
mass of beryllium once again and he found the mistake and gave the correct atomic mass from 13.5 to 9.
Similarly, the doubtful atomic masses of other elements were corrected with the help of Mendeleev's
periodic table.

5. Mendeleev's periodic table inspired subsequent chemists to discover new elements as he left
some gaps for them.

6. Noble gases could accommodate in a new group in Mendeleev's periodic table without disturbing the
existing order after discovery.

Demerits of Mendeleev's periodic table

1. Position of hydrogen: The correct position of hydrogen could not be assigned in Mendeleev's periodic
table. Sometimes, hydrogen loses electron like group-I elements (Li, Na, K, etc.) and sometimes it gains
electron like those of group VII elements i. e. halogens (F, Cl, Br, etc.). Due to this property of
hydrogen, it can be placed either in group I or group VII.

2. Separation of chemically similar elements: Some chemically similar elements have been placed in
different groups. For example, gold and platinum have many similar chemical properties though they
are placed in separated groups i. e. gold is in group I and platinum in group VIII. Similar defects are
seen in copper and mercury, silver and thallium, barium and lead.

3. Grouping of chemically dissimilar elements: Certain chemically dissimilar elements have been
placed in the same group. For example, less reactive elements like copper, silver and gold have been
placed along with highly electropositive or more reactive alkali metals like lithium, sodium and
potassium in group I. Their properties are quite different in many aspects.
4. Similarly, manganese (Mn) is chemically different from halogens like chlorine (Cl),
fluorine (F), bromine (Br) and iodine (I) but all are placed in the same group.

5. Position of isotopes: Isotopes are atoms of same elements which have the same atomic numbers, similar
chemical properties but different atomic masses due to difference in the number of neutrons present in their
nucleus. On the basis of his periodic law, isotopes of elements must have separate places in the periodic table.

But Mendeleev could not give a separated position for isotopes in his periodic table. For example, two isotopes of
chlorine 35Cl and 37Cl should have different places but is assigned only one place.

6. Anomalous pair of elements: Certain elements having higher atomic weights have been placed
wrongly before those with less atomic weights and they do not follow the periodic law. For example,
argon (Ar) having higher atomic weight (39.9) has been placed before potassium (K) having lower
atomic weight (39.1). Similarly, cobalt (27Co58.9) has been placed before nickel (28Ni58.6).

7. Position of lanthanides and actinides: The fourteen elements from atomic numbers 58 cerium (Ce) to
71 lutetium (Lu) that follow lanthanum are termed as lanthanides (rare earth elements).These elements

have similar properties with that of lanthanum (La57). The elements from atomic numbers 90 thorium (Th)
to 103 lawrencium (Lr) that follow actinium are termed as actinides (trans-uranium elements).
These elements have similar properties with that of actinium (Ac89). Mendeleev could not arrange
them properly.
8. Position of metals non-metals and metalloids: No attempt had been made to place metals, non-metals

and metalloids separately in the periodic table. Most of the metals, non-metals and metalloids are
placed in some group. Three elements such as Fe, Co and Ni are placed in the same group VIII. If these
elements were separated, the periodic functions of Mendeleev's periodic table are lost.

♠Mendeleev's periodic table was not able to explain the atomic properties of the elements like valency,
metallic characters, reactivity, ionization potential, etc

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The modern periodic Table

The modern periodic law states, "the physical and the chemical properties of elements are the periodic
functions of their atomic numbers." This means that if the elements are arranged in accordance with the
increasing atomic number, then the elements with similar properties are repeated after a regular interval.
The chart or table that is obtained when elements are arranged in the increasing order of their atomic
numbers is called modern periodic table.

Based on the modern periodic law Bohr constructed a long form of periodic table or modern periodic table.

Advantages of the modern periodic table

The following are the advantages of the modern periodic table over Mendeleev's periodic table:

1. The wrong position of some elements like argon and potassium, cobalt and nickel due to atomic
weights, has been solved by arranging the elements in the order of increasing atomic number without
changing their own places.

2. The isotopes of the same element have the same atomic numbers. Therefore, they find the same
position in the modern periodic table.

3. The position of hydrogen in the periodic table has not yet been solved completely due to its
characteristics of group 1 and group 17. Hydrogen is placed above the alkali metals in the group IA
because of the resemblance in their electronic configuration. However, hydrogen is not regarded as
alkali metals. This makes hydrogen a unique element.

4. The groups of the modern periodic table are divided into sub-groups A and B and treated as separate,
group A and group B because the elements of group A differ in properties from the elements of group B.
It makes the study of elements specific and easier.

5. The elements of groups VIII/18 of Mendeleev's periodic table (iron, cobalt and nickel) are placed
between group IIA/2 and IIIA/13 as a sub-group B.

6. It separates metals from non-metals.

7. The representative and transition elements have been separated in the modern periodic table.

8. In Mendeleev's periodic table, there are eight groups, so it was compulsion to keep alkali metals and
coinage metals in the same cell on the basis of their valency i. e. 1. But in the modern periodic table, the
reactive alkali metals are placed in group IA/1 while the less reactive coinage metals (Cu, Ag and Au)
are placed in group IB/11.

Characteristics of modern periodic table

1. Elements are arranged on the basis of their increasing atomic numbers in the modern periodic table.

2. There are seven periods and 18 groups in the modern periodic table.

3. The strong metals like alkali metals and alkaline earth metals are placed on the left side of transition
elements and non-metals at the right-side and metalloids between metals and non-metals.

4. The inert gases like He, Ne, Ar, Kr, Xn and Rn are kept in the zero group at the extreme right side of
the table.

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5. In the modern periodic table, the elements of lanthanides and actinides series which belong to group
III B/13 are kept below the main table in two separate rows.

6. All the groups are divided into two sub-groups, A and B except zero and VIII groups in the
modern periodic table.

7. The modern periodic table is made up of a series of rectangles. Each rectangle gives us four important
information about elements.

Periods and groups of modern periodic table

Periods

The periodic table consists of seven horizontal rows of the elements called periods. In a period of the periodic
table, elements having gradual change in properties are placed one after another.

S.N Period No. No. of elements Types of periods

1 1 2 very short period

2 2 and 3 8/8 short period

3 4 and 5 18/18 long period

4 6 and 7 32/32 very long period

All the elements in a period show a uniform gradation in their properties from left to right and it is also known as
horizontal relationship in the periodic table.

Characteristics of period

1. Valence electrons: All the elements of a period have different valence electrons in their valence shell. The
number of valence electrons increases from one to eight on moving from left to right. But the number of
shell is the same for all the elements present in the same period. For example, in period 2, the valence
electron of lithium (Li) is 1, beryllium (Be) 2, boron (B) 3, carbon (C) 4 and so on.

2. Valency: The elements in the same period have different valencies. Across a period from left to right, the
valency first increases from 1 to 4, then decreases to 0. This is

shown for the elements of the third period.

Third period Na Mg Al Si PS Cl Ar
Valency –3 –2 10
+1 +2 +3 +4

3. Atomic size: Across a period from left to right, the atomic size decreases as the atomic number increases.
This is because more electrons are added to the outer shell, and the nuclear charge also increases with the
atomic number. As a result, nucleus greatly attracts the electrons nearer to it. Due to this, the outermost
shell contracts and the atomic size decreases. The variations of atomic size of all the elements of period 2 are
given below:

Elements Li Be B CNO F
Atomic size (nm) 0.125 0.09
0.08 0.077 0.075 0.074 0.072

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4. Metallic character: The metallic character decreases as the atomic number increases while moving from left
to right in a period. For example, in period 2, lithium and beryllium are metals, boron is a metalloid while
nitrogen, carbon, oxygen, fluorine and neon are non-metals.

5. Reactivity: The reactivity decreases on moving from left to right in a period of the group IA/1 and IIA/2 to
IVA/14 and after that it increases up to group VIIA/17. Finally, the elements of group zero (0)/18 are inactive or
inert. For example, in period 2, lithium is more reactive than beryllium because the single valence electron of
lithium is attracted by the protons of the nucleus, while 4 protons of its nucleus attract 2 valence electrons of
beryllium. Therefore, it is easier to remove a valence electron from lithium atom than that of beryllium. Thus, in
a period, the reactivity of an element decreases on moving from left to right from group IA/1 up to group
IVA/14 and vice-versa.

Groups

The elements with similar characteristics arranged in the vertical columns of the periodic table are
called groups. There are nine groups numbered by the Roman number from I to VIII and Zero (0). Thus,
there are 18 groups in the long from of the periodic table. The group number to which an element belongs
to in the periodic table is equal to the number of electrons present in the outermost shell of that atom of
element. However, this is valid only for the elements of the sub-group A. The elements of a group have the
same number of electrons in their valance shells and hence show similarity in chemical properties.

Characteristics of groups

The elements of a group have similar chemical properties and show a regular trend in their other properties.
This is also known as vertical relationship in the periodic table.

Features of Elements in a Group

1. Valence electrons: The electrons present in the valence shell of an atom of the element are called valence
electrons The number of valence electrons remains the same in a group. For example, the atoms of all the
elements of group IIA have two electrons in their outermost shell.

Electronic configuration

Elements of sub-group IIA
KLM N

Beryllium (Be) 22

Magnesium (Mg) 28 2

Calcium (Ca) 28 8 2

2. Valency: Valency is defined as the total number of electrons gained or lost or shared by atoms during a chemical
reaction. All the elements in a group have same valency because they have equal number of valence electrons in the
same group. The elements of group

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1. Atomic size: The distance between nucleus and the valence shell of an isolated gaseous atom is called atomic
size or atomic radii of atoms. The size of atoms increases uniformly on moving top to bottom in a group of
the modern periodic table. This is because a new shell is being added as one goes down the group. It is
measured in Pico meter (Pm) [1m=1012pm]. For example, in the group IA of the periodic table, the atomic
radius of lithium, sodium potassium, rubidium, caesium and francium are 123, 157, 202, 216, 235 and 256
respectively.

2. Metallic character: The metallic character is the tendency of an atom of an element to lose one or more
elements. The metallic character increases as we move from top to bottom in a group due to increase in
atomic size and decrease in the force of attraction between the nucleus and the electrons of the valence
shell. For example, sodium is more reactive than lithium.

3. Non-metallic character: It is the tendency of an atom to gain one or more electrons in the valency shell of the atom of an
element when supplied with energy. The non-metallic character decreases on moving from top to bottom in a group
due to increase of atomic size of the elements. For example, fluorine is the most reactive element of group VIIA.

Difference between periods and groups

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Position of different types of elements in modern periodic table

1. Hydrogen: Hydrogen has one valence electron in its valence shell. It has an equal chance to give the
valence electron to others or take one electron from others to become stable. When hydrogen gives its
valence electron to others, it becomes positively charged (H+) and behaves like the metals of group IA.
Like group VIIA elements i. e.
halogens it can gain one electron from other atoms. In this way, its position is justified in both group IA and
group VIIA of the periodic table. However, it is placed at the top of the group IA due to its least atomic
number i. e. l.

2. Representative elements: The first two groups on the left (group 1 and 2) and the last six groups on the
right (group 13 to 18) involve the fillings s- and p- orbitals with elements. These groups 1, 2, 13, 14, 15, 16, 17
and 18 represent the main groups of the periodic table. The elements present in these groups are known as
normal or representative elements.

3. Group IA or 1: The elements belonging to group IA are lithium (Li), sodium (Na), potassium (K),
rubidium (Rb), caesium (Cs) and francium (Fr). Atoms of these elements have only one valence
electron in their valence shell. They lose one electron to form monovalent cation like Li+, Na+, K+, etc.
These elements are also called alkali metals because they react violently with water and produce their
hydroxides which are strongly basic. These elements are shiny, soft and very reactive.

4. Group IIA or 2: The elements belonging to this group are beryllium (Be), magnesium (Mg), calcium (Ca),
strontium (Sr), barium (Ba) and radium (Ra). Atoms of these elements have two electrons in their valence
shell. They lose two valency electrons to form bivalent cations like Be++, Mg++, etc. These elements are
called alkaline earth metals because they form basic hydroxides which are less soluble in water.
Group VIA or 16: The elements of this group are oxygen (O), sulphur (S), selenium (Se), tellurium (Te) and
polonium (Po). These elements are also termed as chalcogens (Gk. chalkos meaning copper) because these
elements are found combined with copper in nature.

5. Group VIIA or 17: The elements of this group are fluorine (F), chlorine (Cl), bromine
(Br), iodine (I) and astatine (At). They have seven valence electrons in their valence shell. They gain one electron
from group IA elements to form a negative ion. These elements are called halogens because they form salts with
group IA elements e.g. NaCl. These elements are highly toxic in high concentration.

6. Lanthanides and actinides: There are two more rows at the bottom of the main body of the modern
periodic table. These two series, each having 14 elements are lanthanides and actinides. The
lanthanides and actinides together are inner-transition elements or rare earth metals. They are also
known as f-block elements because they are the series of elements in which 4f (sub-shell) and 5f are
progressively filled. These two series of f-block elements are placed at the bottom of two separate
rows. This helps to save space and avoid undue sidewise expansion of the periodic table. All the
elements of each series of f-block have similar properties. The lanthanides form the inner transition
complex compounds whereas the actinides include the radioactive elements.

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Reactivity of metals

The elements of group IA, IIA and IIIA are metals. The reactivity of metals increases on moving from top to
bottom in a group of the periodic table.

As we move from top to bottom in a group of metals in the periodic table, the atomic size increases
with the addition of the number of shells and the force of attraction between the nucleus and valence shell
decreases. Due to this, the bigger atom(s) can lose the valance electron(s) more easily than the smaller
atom(s). Hence, the tendency of losing the valence electron is more.

Why is potassium more reactive than the sodium?

Both sodium and potassium are alkali metals which belong to group IA in the periodic table. They contain one valance
electron in their valence shell. Since the atomic size of sodium is smaller than the atomic size of potassium, the force of
attraction between the nucleus and valence shell of potassium is less than that of sodium. So, potassium loses its valence
electron more easily than sodium. Hence, potassium is highly reactive compared to sodium.

Reactivity of non-metals

The elements of group VA/15, VIA/16 and VIIA/17 are non-metals. As we move from top to bottom in a
group of non-metals, the atomic size increases with the addition of the number of shells and the force of
attraction between the nucleus and valence shell decreases. And it is exactly opposite in Metals.

Why is fluorine more reactive than chlorine?
Both fluorine and chlorine are non-metals and belong to group VIIA of the periodic table. The reactivity of the
non-metallic elements of group VIIA decreases as we go from top to bottom.

Sub-shells

The atom, having two electrons in its valence shell (only one orbit), is called a duplet e.g. helium. The atom,
which contains 8 electrons in its valence shell, is called an octet e.g. neon, argon, krypton, etc

All the sub-shells are represented by the letters s, p, d and f. The maximum number of electrons in each
sub-shell is given below:

The s - sub - shell has got maximum 2 electrons.
The p - sub - shell has got maximum 6 electrons.
The d - sub - shell has got maximum 10 electrons.
The f - sub - shell has got maximum 14 electrons.
The region around the nucleus where the probability of finding electrons is maximum is called sub - shell.

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Electronic configuration and its symbolic notation

The systematic representation for the distribution of electrons in different energy levels of an atom is called
electronic configuration

Aufbau principle

Aufbau principle states that "electrons occupy the subshells of minimum energy first and then they occupy the
subshells of maximum energy." This means, electrons enter the subshells having the lowest energy first.
The orbital will be filled by electrons in the following sequence.

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p subshells being filled

Blocks of modern periodic table

According to the arrangement of electrons in the sub-shell, the elements in the periodic table are divided
into 4 blocks. They are s-block, p-block, d-block and f-block.

s-block

The s-block includes the elements of groups IA (alkali metals) and IIA (alkali earth metals). They are present at
the left side of the periodic table.

p-block

The p-block (13), (14), (15), (16), (17), (18) metals, metalloids, non-metals and inert gases which are present in
groups IIIA, IVA, VA, VIA, VIIA and group 0. It is present at the right side of the modern periodic table.

d-block

The d-block includes the elements of sub-groups III B, IVB, VB, VIB, VIIB, VIII, IB (groups 3 to 12) and IIB.
The d-block elements are also called transitional elements because they lie in the middle of the periodic
table between s-block and p-block. All of them are metals.

f-block

The elements of lanthanide and actinide series are called f-block elements. They are also called inner-transition
elements because, while moving from the transition elements of group IIIB to group IVB, we have to transit
through these elements.

1. The elements of group IA (Li, Na, K, Rb, Cs, Fr) are known as reactive metals because they all have a single
valence electron due to which they can easily lose the single electron to other to become stable.

2. The elements of group VIIA/17, are called halogens as they form salt by reacting with metals.

3. In group IA/1, IIA/2 and IIIA/13 reactivity of metals increases from top to bottom. It is because these metals
are electro-positive and they lose valence electron(s) easily from top to bottom in a group due to their
increasing atomic size and decreases the nuclear attraction to the valence shell.

4. In group VA/15, VIA/16 and VIIA/17, reactivity of elements decreases from top to bottom. It is because they
contain non-metals which are electronegative and the tendency of gaining foreign electron(s) decreases
from top to bottom in a group due to their increasing atomic size and decreases the nuclear attraction to the
valence shell.

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5. Atomic size decreases from left to right in a period. It is because of increasing valence electrons from
left to right. As the number of electrons and protons increases, the attraction between the nucleus and
valence electron also increases which makes the last orbit compressed towards the nucleus and the
atomic size decreases.

6. Argon is a noble gas. It is because argon is an octet i. e. it has 8 electrons in its last orbit. Due to this
reason, the element does not combine with others in normal conditions.

7. The elements of group 3 to 12 are called transition elements because they represent change or transition
in character from reactive metals i. e. the elements of group 1 and
2 on the left side to non-metals i. e. the elements of group 13 to 18 on the right side.

Things to remember.

1. The systematic arrangement of elements on the basis of similarities of electronic configuration into
groups and periods is called the periodic table.

2. Mendeleev's periodic law states that ''physical and chemical properties of elements are the
periodic functions of their atomic weights''.

3. The horizontal rows of the periodic table are called periods.

4. The vertical columns of the periodic table are called groups. All the elements of a group have
similar chemical properties as they have the same valency.

5. The modern periodic table is also called the long form of the periodic table.

6. The modern periodic law states that ''the physical and the chemical properties of elements are
periodic functions of their atomic numbers''.

7. The classification of elements based on the modern periodic law is called modern periodic table.

8. There are seven periods and 18 groups in the modern periodic table.

9. The members of group IA/1 are called alkali metals because they dissolve in water, forming
metal hydroxides or alkalis, and hydrogen gas is evolved.

10. Inert gases like helium, neon, argon, krypton, xenon and radon have zero valency and they
have no capacity to combine with other elements.

11. The atoms which contain 8 electrons in their valence shell are called octets.

12. The atoms of the same element having equal atomic number but a different mass number are called
isotopes. The nuclei of the isotopes contain equal number of protons but a different number of neutrons.

13. Valency and atomic size determine the reactivity of elements. The elements of lower valency are more
reactive than those which have more valency. Metals with a big atomic size and non-metals with a small
atomic size are more active than others.

14. In a group from top to bottom reactivity of metals increases reactivity of non-metals decreases.

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