Structure of Atom
Electron arrangements of the first 20 elements in the
NOTE: Nucleus of an atom should be shown with
a plus sign in an atomic structure diagram.
The symbol of some elements are
derived from their Latin Names
NAME OF THE ELEMENT SYMBOL OF THE ELEMENT Latin name of the Element
Sodium Na Natrium
Potassium K Kalium
Iron Cu Cuprum
Copper Ag Argentum
Silver Sn Stannum
Tin Au Aurum
Tungsten Hg Hydrargyrum
Valence shell is the
outermost shell of an atom
having electrons in it.
In the given example,
M shell is the valence shell
ELECTRONS IN THE OUTERMOST SHELL /
K shell L shell
The electrons present in the outermost or valence shell of an atom
are called valence electrons.
The number of atoms present in a molecule of an
element or compound is known as its atomicity.
OCTET RULE – The octet rule refers to the tendency
of atoms to prefer to have eight electrons in the
❑ When atoms have less than 8 electrons in their valence
shell, then they tend to loose, gain or share electrons in
order to have 8 electrons in their valence shell. Thus
forming stable compounds.
An ion is a charged particle. It is charged because it
contains unequal number of protons and electrons.
TYPES OF IONS
FORMATION OF CATION
No. of protons = 11 No. of protons = 11
No. of electrons = 11 No. of electrons = 10
No. of protons = No. of electrons No. of protons > No. of electrons
A cation is a positively charged ion where number of protons is
greater than the number of electrons.
FORMATION OF ANION
No. of protons = 17 No. of protons = 17
No. of electrons = 17 No. of electrons = 18
No. of protons = No. of electrons No. of protons < No. of electrons
An anion is a negatively charged ion where number of electrons is
greater than the number of protons.
A trick to remember…
Positively charged Negatively charged
It has more number of protons It has more number of electrons
than electrons than protons
SOME EXAMPLES SOME EXAMPLES
OF CATIONS: OF ANIONS:
Valency is the
combining capacity of
How to calculate valency?
• Atoms having less than or equal to 4 valence electrons:
To calculate, valency = Number of valence electrons
Example: Aluminium has 3 valence electrons. So,
• Atoms having more than 4 valence electrons:
To calculate, valency = 8- number of valence electrons
Example: Chlorine has 7 valence electrons. So, valency =
THINK AND ANSWER.
Q. Calculate the valency of potassium (valence electron
= 1) and calcium (valence electrons= 2).
Ans- Valency = Number of valence electrons
So, potassium has 1 valence electron. So, valency= 1
Calcium has 2 valence electrons. So, valency = 2
Q. Give reasons:
a) If an atom has 2 electrons (except He) in its outer shell, it has a
tendency to loose electrons.
Ans: An atom is stable if it has 8 electrons in its valence shell
therefore in order to get stable electronic configuration atom loose
2 electrons rather than gaining 6 electrons.
b) Electronegative ions are formed when an atom has atomic number
Z as 17.
Ans: Electronic configuration of an atom with atomic number 17 is 2,
8, 7. Atom will gain one electron to become stable and hence
negative ions are formed.
c) Define Octet rule.
Ans: The octet rule refers to the tendency of atoms to prefer to have
8 electrons in the valence shell.