Structure of an Atom

Structure of Atom

Distribution of
electrons in
energy levels

Electron arrangements of the first 20 elements in the
periodic table

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
Fe Ferrum
Iron Cu Cuprum
Copper Ag Argentum
Silver Sn Stannum
W Wolfram
Tin Au Aurum
Tungsten Hg Hydrargyrum
Pb Plumbum
Gold
Mercury

Lead

OUTERMOST HYDROGEN
SHELL
K shell

OXYGEN

L shell

VALENCE SHELL
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 /
VALENCE SHELL

HYDROGEN OXYGEN

K shell L shell
ONE SIX

VALENCE ELECTRONS:
The electrons present in the outermost or valence shell of an atom
are called valence electrons.



ATOMICITY

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
valence shell.

❑ 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.

Outermost

ION

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…

DIFFERENCE

CATION ANION

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

Valency is the
combining capacity of

an element.

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,
valency= 3

• Atoms having more than 4 valence electrons:
To calculate, valency = 8- number of valence electrons
Example: Chlorine has 7 valence electrons. So, valency =
8-7 =1

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

ASSIGNMENT BOOKLET

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.