VSEPR stands for Valence Shell Electron Pair Repulsion. The core postulate is that valence electron domains (groups of electrons) around a central atom repel each other, and adopt the arrangement that minimizes total repulsion between all electron domains.
2. Electron Domain vs Molecular Geometry★★★☆☆⏱ 7 min
VSEPR distinguishes two types of geometry: electron domain geometry describes the arrangement of all electron domains (bonding and lone pairs), while molecular geometry describes the arrangement of only the bonded atoms. Lone pairs are not included when naming molecular geometry.
3. Basic Orbital Hybridization★★★☆☆⏱ 5 min
VSEPR predicts molecular shape but cannot explain why bond angles match experimental observation. For example, carbon in methane has four identical bonds, even though ground state carbon only has 2 unpaired electrons available for bonding. Hybridization resolves this by mixing valence atomic orbitals to form new, identical hybrid orbitals.
The number of hybrid orbitals formed equals the number of atomic orbitals mixed.
Hybrid orbitals form sigma ($\sigma$) bonds or hold lone pairs.
Unhybridized p orbitals form pi ($\pi$) bonds in multiple bonds.
4. Relating Steric Number to Hybridization★★★☆☆⏱ 3 min
The steric number (equal to the total number of electron domains around the central atom) directly gives the hybridization of the central atom for most small molecules with octet-compliant central atoms.
Common Pitfalls
Why: VSEPR counts all multiple bonds as a single electron domain, since they occupy the same region of space between two atoms
Why: Exam questions almost always ask for molecular geometry (arrangement of atoms), which excludes lone pairs from the shape description
Why: Lone pairs exert greater repulsion than bonding pairs, compressing bond angles below the ideal value
Why: A central atom with 3 bonds and 1 lone pair has 4 electron domains, so it is $sp^3$ hybridized, not $sp^2$