Shapes of molecules

MDCAT Chemistry · Chemistry · Chemical bonding. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

VSEPR theory says that the electron pairs around a central atom repel each other and move as far apart as possible. Count the bond pairs and lone pairs: their number fixes the shape. Lone pairs repel more strongly than bond pairs, so they squeeze the bond angles. A double or triple bond counts as one region.

Hybridisation explains how the bonds form: orbitals of one atom mix to give equal hybrid orbitals (sp, sp², sp³) that point in the VSEPR directions.

Hybrid orbitals overlap head-on to make sigma bonds. Unhybridised p orbitals overlap sideways to make pi bonds. These simple models have limits: they cannot explain why O₂ is paramagnetic, but molecular orbital theory can.

Rules to remember

Repulsion: lone pair–lone pair > lone pair–bond pair > bond pair–bond pair.
2 regions: linear, 180°, sp (BeCl₂, CO₂). 3 regions: trigonal planar, 120°, sp² (BF₃). 4 regions: tetrahedral, 109.5°, sp³ (CH₄).
5 bond pairs: trigonal bipyramidal, 90° and 120° (PCl₅). 6 bond pairs: octahedral, 90° (SF₆).
Four pairs with lone pairs: NH₃ (one lone pair) is trigonal pyramidal, 107.5°; H₂O (two lone pairs) is bent, 104.5°.
s-character: sp 50%, sp² 33%, sp³ 25%.
Single bond = 1σ; double bond = 1σ + 1π; triple bond = 1σ + 2π. A σ bond is stronger because head-on overlap is greater; π electron density lies above and below the line joining the nuclei.
Molecular orbital theory: O₂ has two unpaired electrons in antibonding π orbitals, so it is paramagnetic, with bond order 2.

Common mistake

Students count only the bonded atoms and forget the lone pairs. NH₃ has three bonds but four electron pairs, so it is pyramidal and sp³, not trigonal planar.

Worked example

Predict the shape and bond angle of the amide ion, NH₂⁻.

Electrons around N: 5 of its own + 2 from the two H atoms + 1 for the negative charge = 8 electrons = 4 pairs.
Two are bond pairs and two are lone pairs. Four pairs are arranged tetrahedrally, so N is sp³ hybridised.
The shape counts only the atoms: bent (angular), like water. The two lone pairs push the bonds together, so the angle is less than 109.5°.

Answer: Bent (angular), with an H–N–H angle of about 105°.

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