Read Packing and Defects Before Counting Formula Units
Solid-state questions split into three families — reading an empirical formula off a packing diagram, identifying which defect changes density and which doesn't, and applications like zeolites that lean on pore/cavity geometry rather than pure crystallography.
Crystal Structures, Defects & Applications is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Empirical formula from a packing diagram
For a structure like perovskite, where one ion type forms a close-packed lattice, a second occupies an interstitial hole, and charges are omitted, count atoms per unit cell by their fractional occupancy (corner, face, edge, body-centre) to get the ratio, then write the simplest formula — for calcium/oxide in ccp with titanium in an octahedral hole, this gives CaTiO₃.
Schottky vs Frenkel defects, and what changes density
A Schottky defect (paired cation and anion vacancies) lowers density since mass is lost from the lattice without a volume change. A Frenkel defect (an ion displaced into an interstitial site) does NOT change density, since no ions leave the lattice — don't assume every defect lowers density the same way.
Zeolites: pores decide the chemistry
Zeolites act as molecular sieves and industrial catalysts precisely because their pore and cavity size/shape controls which reactant molecules can enter and react — aluminium substitution for silicon, cracking of hydrocarbons, and isomerisation reactions in petrochemistry all trace back to this pore-selectivity property.
OlympIQ exam lens
- ✓Read formula ratios off a packing diagram from fractional atom occupancy per unit cell, not a visual headcount.
- ✓Schottky defects lower density (mass lost, volume unchanged); Frenkel defects leave density unchanged (no ions actually leave the lattice).
- ✓Zeolite chemistry (cracking, isomerisation, molecular sieving) is controlled by pore and cavity size, not just composition.
Don't assume every point defect lowers density the way a Schottky defect does — a Frenkel defect just relocates an ion within the lattice, so no mass is actually lost and density stays the same.
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