Topic 2.7 VSEPR and Bond Hybridization: use VSEPR theory to predict molecular geometry and bond angles, assign the hybridization of the central atom, and relate geometry to molecular polarity.

What this topic is asking

The College Board (Topic 2.7) wants you to use VSEPR theory to predict a molecule's shape and bond angles from its Lewis diagram, to assign the hybridization of the central atom, and to use the shape to decide whether the molecule is polar or nonpolar. This topic turns the flat Lewis diagram into a three-dimensional structure with real consequences.

VSEPR theory

So the first job is to count electron domains on the central atom from the Lewis diagram: each single, double or triple bond is one domain, and each lone pair is one domain.

Geometry and bond angles

Lone pairs matter twice. First, they occupy a domain, so a molecule with lone pairs has a different molecular shape from its electron-domain geometry. Second, lone pairs repel more strongly than bonding pairs (they are held by only one nucleus and spread out more), so they squeeze the bonding pairs together and reduce the bond angle below the ideal. This is why water's H-O-H angle is about $104.5^\circ$ rather than $109.5^\circ$.

Hybridization

The number of electron domains also tells you the hybridization of the central atom, which is the mixing of atomic orbitals that produces the right number of equivalent bonding directions:

• 2 domains: $sp$ (linear).
• 3 domains: $sp^2$ (trigonal planar).
• 4 domains: $sp^3$ (tetrahedral).

Hybridization is a model that makes the orbital picture consistent with the observed geometry. Count the domains, then read off the hybridization directly.

Shape determines polarity

A molecule is polar if it has a net dipole moment, and nonpolar if the individual bond dipoles cancel. Crucially, a molecule can have polar bonds yet be nonpolar overall if its shape is symmetric. Carbon dioxide ($\text{O}=\text{C}=\text{O}$) is linear, so its two equal C=O dipoles point in opposite directions and cancel, making the molecule nonpolar. Water is bent, so its two O-H dipoles do not cancel and the molecule is polar. This is the payoff of the whole unit: the Lewis diagram (Topic 2.5) gives the domains, VSEPR gives the shape, and the shape, combined with bond polarity (Topic 2.1), gives the molecular polarity that controls intermolecular forces, solubility and boiling point in later units. Treating shape and polarity as one connected chain, rather than separate facts, is exactly the reasoning the College Board rewards.

Try this

Q1. Predict the molecular shape and bond angle of $\text{BF}_3$ (three bonding regions, no lone pairs on boron). [2 points]

• Cue. Trigonal planar with $120^\circ$ bond angles.

Q2. State the hybridization of carbon in methane, $\text{CH}_4$. [1 point]

• Cue. $sp^3$ (four electron domains).