Rotate the rock-salt, caesium chloride, zinc blende and fluorite structures, count coordination numbers and test the radius-ratio rule.
Last updated: 26 October 2026
Yellow rods join one ion to its nearest neighbours, so you can count the coordination number directly. The radius ratio predicts which structure a salt prefers. Syllabus note: unit cells and crystal structures are not a unit in the current JEE Main chemistry topic list on this site. They still appear in older PYQs and some other exams, so check the latest official syllabus before spending time here.
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6 : 6. Each Na⁺ is surrounded by 6 Cl⁻ at the corners of an octahedron, and vice versa.
Cs⁺ is large, so the radius ratio is above 0.732 and the cation fits in a cubic hole touching 8 anions.
