Read Isostructural Pairs by Electron Domain Count, Not by Formula
Isostructural species share the same shape — which means the same electron domain count and the same lone-pair-vs-bonding-pair split — regardless of how different their formulas look on paper.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Same shape doesn't require the same formula type
BH₄⁻ and NH₄⁺ are isostructural (both tetrahedral, 4 bonding domains, 0 lone pairs) despite one being anionic and one cationic — count domains on the central atom, not overall charge or atom count, to test isostructural claims.
Isoelectronic is a stronger condition than isostructural
Isoelectronic species (same total electron count) are very often also isostructural, since matching electron count usually means matching electron-domain arrangement too — but always verify both conditions independently, since isoelectronic species built from different central atoms can occasionally still differ in fine structural detail.
Checking a proposed pair systematically
For any proposed isostructural pair, draw out (or count) each species' central-atom electron domains separately — SiF₄ and SF₄ are NOT isostructural (4 domains/tetrahedral vs 5 domains/see-saw with a lone pair) despite looking like a minimal one-atom swap on paper.
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- ✓Isostructural means matching electron domain count AND matching lone-pair/bonding-pair split on the central atom.
- ✓Overall molecular charge doesn't need to match for two species to be isostructural — only domain geometry does.
- ✓Isoelectronic species are often isostructural too, but verify both conditions independently rather than assuming one implies the other.
- ✓A one-atom formula swap (like SiF₄ → SF₄) can change domain count entirely — don't assume structural similarity from formula similarity.
Don't judge isostructural pairs by formula pattern-matching — always count electron domains on the central atom of each species independently, since a minor atom swap can add or remove a lone pair and completely change the shape.
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