2.7 VSEPR and Bond Hybridization

VSEPR predicts molecular geometry

Valence Shell Electron Pair Repulsion (VSEPR) theory states that electron pairs (bonding and lone) around a central atom arrange themselves to minimize repulsion. Counting electron domains (bonds + lone pairs) determines the electron-domain geometry, while lone pairs push bonded atoms closer together, giving the observed molecular geometry (e.g., linear, trigonal planar, tetrahedral, trigonal pyramidal, bent).

Hybridization explains bonding orbitals

Hybridization mixes atomic orbitals to match a molecule's geometry. Two domains → sp (linear); three domains → sp² (trigonal planar); four domains → sp³ (tetrahedral). Hybrid orbitals form sigma (σ) bonds, while unhybridized p-orbitals form pi (π) bonds in double and triple bonds.

Sigma vs. pi bonds

A single bond = 1 σ bond; a double bond = 1 σ + 1 π bond; a triple bond = 1 σ + 2 π bonds. σ bonds allow rotation; π bonds restrict rotation, creating rigid, planar regions in molecules like ethene.

Key terms

VSEPR theory

Electron pairs around a central atom arrange themselves to minimize repulsion, determining molecular shape.

Electron domain

A region of electron density around a central atom, including bonding pairs and lone pairs.

sp³ hybridization

Mixing of one s and three p orbitals to form four equivalent hybrid orbitals; tetrahedral arrangement.

Sigma (σ) bond

A bond formed by direct, head-on overlap of orbitals; allows rotation; present in all single, double, and triple bonds.

Pi (π) bond

A bond formed by side-by-side overlap of unhybridized p-orbitals; restricts rotation; found in double and triple bonds.

Try an AP-style question

What is the molecular geometry of NH₃?

1. Trigonal pyramidal
2. Trigonal planar
3. Bent
4. Tetrahedral