Chapter-Journey Inside the Atom Science Exploration class 9 in english Medium CBSE Notes

Chapter 8. Journey Inside the Atom

This section explains how electrons are arranged in different shells of an atom. It introduces the Bohr–Bury rules, electronic configuration, valence shell, valence electrons and valency. These concepts help us understand why different elements show different chemical properties and how atoms combine to form compounds. 

Electronic Configuration

The arrangement of electrons in different shells or energy levels of an atom is called its electronic configuration. Electrons are filled in the shells according to definite rules so that the atom remains stable. 

Definition

Electronic configuration is the systematic arrangement of electrons in different shells of an atom.

Bohr–Bury Rules

Bohr and Bury proposed simple rules to explain how electrons are distributed in different shells.

Main Rules

• Maximum Capacity of a Shell – The maximum number of electrons in a shell is given by the formula 2n², where n is the shell number.
• Outermost Shell Rule – The outermost shell can have a maximum of 8 electrons. However, the first shell (K-shell) can accommodate only 2 electrons.
• Stepwise Filling – Electrons are filled from the shell nearest to the nucleus towards the outer shells in the order K → L → M → N.

 

Maximum Number of Electrons in Different Shells

Shell	Shell Number (n)	Maximum Electrons (2n²)
K	1	2
L	2	8
M	3	18
N	4	32

Electronic Configuration of Some Elements

Element	Atomic Number	Electronic Configuration
Hydrogen	1	1
Helium	2	2
Lithium	3	2, 1
Carbon	6	2, 4
Oxygen	8	2, 6
Sodium	11	2, 8, 1
Chlorine	17	2, 8, 7

Valence Shell

The outermost shell of an atom is called the valence shell. The electrons present in this shell take part in chemical reactions.

Definition

The outermost occupied shell of an atom is known as the valence shell.

Valence Electrons

The electrons present in the valence shell are called valence electrons. These electrons determine the chemical properties of an element.

Importance of Valence Electrons

• They participate in chemical bonding.
• They determine the valency of an element.
• They influence the chemical behaviour of atoms.

Valency

Atoms become stable by completing their outermost shell. During chemical reactions, they may lose, gain or share electrons. The combining capacity of an atom is called its valency.

Definition

Valency is the combining capacity of an atom, determined by the number of electrons it loses, gains or shares to achieve a stable electronic configuration.

Valency of Some Common Elements

Element	Electronic Configuration	Valency
Hydrogen	1	1
Helium	2	0
Lithium	2,1	1
Carbon	2,4	4
Nitrogen	2,5	3
Oxygen	2,6	2
Sodium	2,8,1	1
Chlorine	2,8,7	1

Importance of Electronic Configuration

• Helps predict the chemical behaviour of elements.
• Explains the formation of chemical bonds.
• Determines the valency of atoms.
• Provides the basis for the arrangement of elements in the periodic table.

ATP Education Concept Builder

Remember the sequence K → L → M → N. Electrons always fill the shell nearest to the nucleus first. The outermost shell decides how an atom behaves chemically, because its valence electrons take part in chemical reactions.

ATP Education Exam Booster

• Memorise the Bohr–Bury rules.
• Remember the formula 2n² for the maximum number of electrons in a shell.
• Learn the electronic configurations of common elements up to atomic number 20.
• Differentiate between valence shell and valence electrons.
• Understand how valency is determined from electronic configuration.
• CBSE competency-based questions frequently ask students to write electronic configurations and determine valency from them.

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