Packing in Solids — Predicted 2026

AI-Predicted Question Angles for UPSC 2026

Based on trend analysis, current affairs, and recurring themes in Packing in Solids.

Compound Formula from Void Occupancy (Complex)

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NEET frequently tests the ability to determine the empirical formula of a compound when one type of atom forms the lattice and another occupies a specific fraction of voids. Future questions might involve more complex fractions of void occupancy (e.g., 1/2 of tetrahedral and 1/4 of octahedral voids) or scenarios where both types of voids are partially occupied. This requires a clear understanding of the number of atoms per unit cell and the number of each type of void.

Density Calculation involving Packing Efficiency

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While direct density calculations are common, a question might indirectly test packing efficiency by providing density and asking for atomic radius or vice-versa, requiring the use of the packing efficiency concept to relate 'a' and 'r'. The formula for density, $\rho = \frac{Z \times M}{a^3 \times N_A}$, combined with the 'a' vs 'r' relationship for a specific lattice, can form a multi-step problem. This tests both packing knowledge and general solid-state calculations.

Identification of Crystal Structure from Properties/Description

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Instead of directly stating 'FCC lattice', a question might describe a metal as having a coordination number of 12 and a packing efficiency of 74%, and then ask about its unit cell type or 'a' vs 'r' relationship. Or, it might describe the stacking sequence (e.g., 'ABCABC... arrangement') and ask to identify the structure. This tests the student's ability to infer the crystal structure from its characteristic properties, requiring a deeper conceptual grasp.