Chemical Bonding

Topics covered: Types of chemical bonds, Lewis structures, bond parameters, VSEPR theory, hybridisation, molecular orbital theory and hydrogen bonding (JEE Main).

1) Chemical Bond

A chemical bond is the force that holds atoms together in a molecule.

• Atoms combine to attain stable electronic configuration.
• Noble gas configuration is the most stable.

2) Octet Rule

• Atoms tend to complete 8 electrons in the valence shell.
• Applicable mainly to second-period elements.

Exceptions:

• Incomplete octet: BF₃, BeCl₂
• Expanded octet: PCl₅, SF₆
• Odd-electron molecules: NO, NO₂

3) Types of Chemical Bonds

(a) Ionic Bond

• Formed by transfer of electrons.
• Exists between metal and non-metal.
• High melting and boiling points.

(b) Covalent Bond

• Formed by sharing of electrons.
• Low melting and boiling points.

(c) Coordinate (Dative) Bond

• Shared pair of electrons donated by one atom.
• Example: NH₄⁺, H₃O⁺

4) Lewis Structures

• Represent valence electrons as dots.
• Helps predict bonding and lone pairs.

5) Bond Parameters

Bond Length:

• Distance between nuclei of bonded atoms.
• Single > Double > Triple (length)

Bond Energy:

• Energy required to break a bond.
• Triple > Double > Single (strength)

Bond Angle:

• Angle between two bonds originating from the same atom.

6) VSEPR Theory

Valence Shell Electron Pair Repulsion theory predicts molecular shape.

• Lone pair–lone pair > lone pair–bond pair > bond pair–bond pair repulsion

Common Shapes:

• Linear – BeCl₂
• Trigonal planar – BF₃
• Tetrahedral – CH₄
• Trigonal pyramidal – NH₃
• Bent – H₂O

7) Hybridisation

Hybridisation: Mixing of atomic orbitals to form equivalent hybrid orbitals.

8) Molecular Orbital Theory (MOT)

Bond Order:

Bond order = (N_b − N_a) / 2

• Higher bond order → stronger bond.
• If bond order = 0, molecule does not exist.

Magnetic Behaviour:

• Paramagnetic: unpaired electrons
• Diamagnetic: all electrons paired

9) Hydrogen Bonding

Attractive force between hydrogen attached to highly electronegative atom (F, O, N).

• Intermolecular hydrogen bonding: HF, H₂O
• Intramolecular hydrogen bonding: o-nitrophenol

Effects:

• Higher boiling point
• Higher viscosity
• Higher solubility in water