Kossel-Lewis Theory and Octet Rule | Class 11 Chemistry

Kossel and Lewis Theory

Class 11 Chemistry • Chemical Bonding

1. Kossel and Lewis Theory

Definition: In 1916, Kossel and G. N. Lewis independently proposed the Electronic Theory of Valence.

According to this theory, atoms combine with each other to attain a stable electronic configuration similar to that of noble gases.

2. Main Idea of the Theory

The atoms tend to achieve a stable electronic configuration by loss, gain or sharing of electrons.

• Kossel explained bond formation mainly by transfer of electrons.
• Lewis explained bond formation mainly by sharing of electrons.
• The electrons present in the outermost shell are called valence electrons.

3. Kossel's Concept

According to Kossel, atoms can attain stable noble gas configuration by loss or gain of electrons.

• Metal atoms generally lose electrons and form positive ions.
• Non-metal atoms generally gain electrons and form negative ions.
• The oppositely charged ions are held together by electrostatic attraction.
• This leads to the formation of an ionic bond.

Example: Formation of NaCl

Na → Na⁺ + e⁻

Cl + e⁻ → Cl⁻

Na⁺ and Cl⁻ attract each other and form NaCl.

4. Lewis's Concept

According to Lewis, atoms can attain stable electronic configuration by sharing electron pairs.

• The shared pair of electrons forms a covalent bond.
• One shared pair represents one covalent bond.
• Two shared pairs represent a double bond.
• Three shared pairs represent a triple bond.

Examples:

H—H   →   Single bond

O=O   →   Double bond

N≡N   →   Triple bond

5. Octet Rule

Octet Rule: Atoms tend to gain, lose or share electrons so that they have eight electrons in their valence shell.

This gives the atom a stable electronic configuration similar to noble gases.

Exception: Hydrogen and lithium can attain stability with two electrons in their outermost shell (duplet).

6. Kossel vs Lewis

Kossel

• Involves transfer of electrons.
• Formation of ions takes place.
• Explains ionic bond.
• Example: NaCl

Lewis

• Involves sharing of electrons.
• Electron pairs are shared.
• Explains covalent bond.
• Example: H₂, Cl₂

7. Examples of Covalent Molecules

Molecule	Bond
H2	Single covalent bond
Cl2	Single covalent bond
O2	Double covalent bond
N2	Triple covalent bond
H2O	Two O—H covalent bonds
CO2	Two double covalent bonds

8. Limitations of Octet Rule

1. Incomplete Octet:
Some atoms in stable molecules have fewer than eight electrons around them.

Examples: BF₃, BeCl₂, LiCl

2. Expanded Octet:
Some atoms can have more than eight electrons around them.

Examples: SF₆, PCl₅, H₂SO₄

3. Odd-electron Molecules:
Molecules containing an odd number of electrons cannot satisfy the octet requirement for all atoms.

Examples: NO, NO₂

4. Molecular Shape:
The octet rule cannot explain the molecular shape or geometry of molecules.

5. Energy and Reactivity:
The octet rule cannot adequately explain differences in energy and reactivity of molecules.

⭐ Important for Examination

Kossel: Transfer of electrons → Ions → Ionic bond

Lewis: Sharing of electrons → Electron pair → Covalent bond

Octet Rule: Atoms tend to attain eight electrons in their valence shell.