VSEPR and Bond Hybridization — AP Chemistry
1. VSEPR: Steric Number and Molecular Geometry ★★☆☆☆ ⏱ 4 min
Valence Shell Electron Pair Repulsion (VSEPR) is the core predictive model for 3D molecular shape, built on the principle that negatively charged electron domains (bonding pairs or lone pairs) repel one another and arrange to minimize total repulsion. Bond hybridization is the complementary quantum mechanical model that explains how atomic orbitals mix to form equivalent hybrid orbitals aligned with VSEPR-predicted shapes, resolving the mismatch between unhybridized atomic orbital orientations and observed molecular geometry.
SN = \text{number of bonding domains } (n) + \text{number of lone pairs on the central atom } (m)
Electron domain geometry describes the arrangement of all electron domains (both bonding and lone pairs), while molecular geometry (the observable shape of the molecule) only describes the arrangement of bonded atoms. Repulsion strength follows a consistent order: lone pair-lone pair repulsion > lone pair-bonding pair repulsion > bonding pair-bonding pair repulsion. This causes deviations from ideal bond angles when lone pairs are present.
Exam tip: Never count multiple bonds as more than one electron domain. Even though double/triple bonds have more electron density, AP exam rules treat them as one domain for steric number and geometry calculations.
2. Bond Hybridization: Correlation to Steric Number ★★★☆☆ ⏱ 4 min
Hybridization is the mixing of valence atomic orbitals (s, p, d) from the central atom to form new, equal-energy hybrid orbitals that align with VSEPR electron domain arrangement, minimizing repulsion. Each hybrid orbital holds exactly one electron domain (either a bonding pair or a lone pair), so the number of hybrid orbitals formed equals the number of atomic orbitals mixed, which equals the steric number of the central atom. Unhybridized p orbitals left over after hybridization form pi bonds: a double bond has 1 sigma (hybrid overlap) + 1 pi (unhybridized p overlap), a triple bond has 1 sigma + 2 pi.
- $SN=2 \rightarrow sp$ (1 s + 1 p mixed, 2 hybrids)
- $SN=3 \rightarrow sp^2$ (1 s + 2 p mixed, 3 hybrids)
- $SN=4 \rightarrow sp^3$ (1 s + 3 p mixed, 4 hybrids)
- $SN=5 \rightarrow sp^3d$ (1 s + 3 p + 1 d, 5 hybrids)
- $SN=6 \rightarrow sp^3d^2$ (1 s + 3 p + 2 d, 6 hybrids)
Exam tip: Always calculate steric number to find hybridization, don’t assume all carbon is $sp^3$ just because it has 4 total covalent bonds. A carbon with a double bond has 3 electron domains, so it is $sp^2$, even with 4 total covalent bonds.
3. VSEPR Exceptions: Expanded Octets and Lone Pair Placement ★★★★☆ ⏱ 3 min
Most small main-group molecules follow basic VSEPR rules, but two key exceptions are commonly tested on the AP exam. First, only central atoms from period 3 or lower can have steric numbers greater than 4 (expanded octets), because they have empty d-orbitals in their valence shell that can participate in hybridization. Period 2 elements (Be, B, C, N, O, F) only have s and p valence orbitals, so they can never have more than 4 electron domains, no exceptions. Second, for SN=5 (trigonal bipyramidal electron domain geometry), lone pairs always occupy equatorial positions rather than axial positions to minimize 90° repulsions.
Exam tip: For SN=5 structures with lone pairs, always place lone pairs equatorial first. Defaulting to axial placement (a common mistake from 2D Lewis drawings) will always lead to an incorrect molecular geometry.
4. AP-Style Concept Check ★★★☆☆ ⏱ 3 min
Common Pitfalls
Why: Students confuse total number of covalent bonds with number of electron domains, since multiple bonds have more total electrons
Why: Students forget that period 2 elements don’t have d-orbitals in their valence shell to mix for hybridization
Why: Students memorize the electron domain geometry for a given steric number, and forget the question asks for the shape of the molecule (only bonded atoms count)
Why: Carbon almost always has 4 total covalent bonds, so students assume all carbon is $sp^3$
Why: Students memorize ideal angles and forget the effect of lone pair repulsion