Read VSEPR as Electron-Pair Repulsion, Not Memorised Shapes
Every VSEPR question is the same underlying task: count electron domains (bonding + lone pairs) around the central atom, place them to minimise repulsion, then read off the molecular shape by ignoring lone pairs in the final description.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Lone pairs occupy equatorial positions preferentially in trigonal bipyramidal systems
In a 5-electron-domain (trigonal bipyramidal) system like SF₄, ClF₃ or BrF₅, lone pairs always occupy equatorial positions first, since equatorial positions have fewer 90° neighbours than axial ones and lone-pair repulsion is minimised there. This single rule predicts SF4's see-saw shape, ClF3's T-shape, and why BrF₅ has zero 90° F-Br-F angles is a common trap — check the actual geometry (square pyramidal, not trigonal bipyramidal) before assuming the 5-domain rule applies.
Xenon compounds follow the same rules with more lone pairs
XeF₂ (3 lone pairs, linear), XeF₄ (2 lone pairs, square planar) and XeF₆ (1 lone pair, distorted octahedral) form a clean progression — each loses one Xe lone pair as one more F is added, and the number of lone pairs directly determines whether the shape distorts from the parent electron-domain geometry.
Bond angle comparisons need one variable changed at a time
Comparing bond angles down a group (NH₃ > PH₃ > AsH₃ > SbH₃, decreasing due to decreasing central-atom electronegativity and less s-character in bonding orbitals) is a different comparison from comparing across substituents on the same central atom (NH₃ vs NF₃, where electronegative F pulls bonding pairs away from N, shrinking the angle despite F being 'smaller') — identify which variable the question is actually changing before predicting direction.
Matching shapes and identifying incorrect shape claims
For match-the-column and 'which shape is correct' questions, work out hybridisation and lone-pair count for EACH species independently rather than pattern-matching to a memorised list — I³⁻, ICl₂⁻, XeF₂ are all AX₂E₃ systems (linear) despite looking structurally different at first glance.
OlympIQ exam lens
- ✓Count total electron domains (bonds + lone pairs) first — shape name only describes where the ATOMS are, not the domains.
- ✓In trigonal bipyramidal (5-domain) systems, lone pairs go equatorial first to minimise 90° repulsions.
- ✓Xenon fluorides XeF2/XeF4/XeF₆ lose one lone pair each as one more F is added, from 3 down to 1.
- ✓Bond angle comparisons down a group and bond angle comparisons across substituents on the same atom follow different underlying causes — don't conflate them.
Don't assume every 5-electron-domain molecule has 90° angles between adjacent domains — BrF₅ is square pyramidal (from an octahedral parent with 1 lone pair), not trigonal bipyramidal, so its F-Br-F angles are close to 90° but the axial-lone-pair-vs-basal distinction changes which specific angle comparisons are valid.
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