VSEPR Theory: MDCAT Chemistry notes

Basic idea

Valence Shell Electron Pair Repulsion (VSEPR) theory predicts the shape of a molecule. The electron pairs in the valence shell of the central atom repel one another, so they arrange themselves as far apart as possible, which keeps repulsion to a minimum. VSEPR explains shapes and bond angles. It does not explain bond energy or how bonds are formed; that is the work of VBT and MOT.

Postulates

1. Both bond pairs and lone pairs around the central atom decide the geometry. Lone pairs are not the only ones that count.
2. The pairs arrange themselves to minimize repulsion.
3. A lone pair is held by only one nucleus, so it spreads out more and repels more strongly. The order of repulsion is: $$\text{lp–lp} > \text{lp–bp} > \text{bp–bp}$$
4. For shape purposes, a double or triple bond counts as one region (one bond pair).
5. Lone pairs squeeze bond angles below the ideal value.

Uses: predicting molecular shapes, and so polarity. The FSc textbook also notes that it helps explain how drug molecules fit and interact with receptors in the body.

Counting electron pairs

Total pairs = bond pairs + lone pairs on the central atom.

• $\mathrm{H_2O}$: O has 6 valence electrons. Two are used in O–H bonds and four are left as 2 lone pairs. So there are 2 bp + 2 lp = 4 pairs, arranged tetrahedrally, and the molecule is bent at 104.5°.
• $\mathrm{NH_3}$: 3 bp + 1 lp = 4 pairs, so the molecule is pyramidal at 107°.
• $\mathrm{CH_4}$: 4 bp + 0 lp, so the molecule is tetrahedral at 109.5°.

Electron-pair geometry

Octet rule and its exceptions

A species has a complete octet when its outer shell holds 8 electrons (2 for the helium-like case). Quick check: count the total electrons and compare with a noble gas.

• $\mathrm{O^{2-}}$ (10 e), $\mathrm{K^+}$ (18 e) and $\mathrm{Ca^{2+}}$ (18 e) are all noble-gas-like and have complete octets.
• A hypothetical $\mathrm{N^{5+}}$ keeps only its 2 inner (1s) electrons and has no valence octet at all.
• Electron-deficient molecules such as $\mathrm{BF_3}$ and $\mathrm{BeCl_2}$ have fewer than 8 electrons round the central atom. Expanded octets occur in $\mathrm{PCl_5}$ and $\mathrm{SF_6}$.

Common MDCAT traps

• The false statement is usually "only lone pairs determine the geometry". Both kinds of pair count.
• Water has 4 electron pairs round O, not 2. Its lone pairs are easy to forget.
• VSEPR gives shapes. It is not a theory of bond energy or bond formation.
• Multiple bonds count as one region, so $\mathrm{CO_2}$ is linear, like $\mathrm{BeCl_2}$.

Quick revision

• Electron pairs repel and move as far apart as possible.
• Repulsion order: lp–lp > lp–bp > bp–bp.
• Four pairs give a tetrahedral arrangement.
• Lone pairs reduce bond angles: 109.5° in $\mathrm{CH_4}$, 107° in $\mathrm{NH_3}$, 104.5° in $\mathrm{H_2O}$.