hybridisation

Meaning: Hybridisation is the mixing of an atom's valence s and p orbitals into equivalent hybrid orbitals that point toward its electron domains. It explains geometries such as the four equal C–H bonds of methane. Hybrid orbitals form sigma bonds and hold lone pairs; unhybridised p orbitals left over form pi bonds. Two domains give sp (linear, 180°), three give sp² (trigonal planar, 120°), and four give sp³ (tetrahedral, about 109.5°).

Equation: Number of hybrid orbitals = number of σ bonds + number of lone pairs, where 2 → sp, 3 → sp², 4 → sp³ (a count, so dimensionless; bond angles are in degrees).

On the MCAT: You are given a structure and asked to assign the hybridisation of a labeled atom, then predict geometry, bond angle, or the number of sigma and pi bonds. A higher s character means shorter, stronger bonds and a more acidic C–H, so an alkyne C–H (sp) is more acidic than an alkene or alkane C–H.

Common confusion: Pi bonds are not extra domains. Each carbon in ethene has three sigma bonds, so it is sp², not sp³. Lone pairs do count: nitrogen in NH₃ is sp³, with a bond angle near 107°. An atom with a lone pair next to a pi system, such as amide nitrogen, is often sp² so the lone pair can join resonance.