Valence Bond Theory (VBT)

CompleteFundamentals of Chemistry

VBT treats a bond as formed by the overlap of atomic orbitals holding one electron each.

Full notes

Definition: According to VBT, a covalent bond forms when atomic orbitals of two atoms overlap so that each orbital contributes one electron of opposite spin; the overlapping electrons pair up in the region between nuclei.

1. Rules of VBT

Each atom keeps its individual orbitals (orbitals are not 'lost').
Overlap of half-filled orbitals forms the bond.
Bond strength ∝ extent of overlap.
Orbitals of compatible symmetry overlap effectively.
Pauli principle — two shared electrons have opposite spins.

2. Sigma (σ) and pi (π) bonds

End-on overlap (s–s, s–p, p–p head on) gives a σ bond — free rotation is possible around it. Sideways overlap of two parallel p orbitals gives a π bond — rotation is restricted because it breaks the π bond. A double bond is 1σ + 1π; a triple bond is 1σ + 2π. π bonds are weaker than σ bonds and are the site of most addition reactions.

Property	σ bond	π bond
Formation	End-on overlap	Sidewise overlap
Strength	Stronger	Weaker
Rotation	Free	Restricted
Reactivity	Less reactive	More reactive (additions)

Note: Exam point: 'explain why alkenes are more reactive than alkanes' — answer uses the weaker, exposed π bond.

Exam notes

VBT: overlap of half-filled orbitals → electron pair. σ = end-on; π = sideways. Double bond = σ+π, triple = σ+2π.

Important points

VBT explains geometry via hybridization and fails where MOT needed (O₂ paramagnetism).

Common mistakes

Thinking a double bond is just 'two bonds of equal strength' — one is σ, one is π.

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