Close Packed Structures — Predicted 2026

AI-Predicted Question Angles for UPSC 2026

Based on trend analysis, current affairs, and recurring themes in Close Packed Structures.

Compound Formula based on Void Occupancy

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This is historically the most frequently tested aspect. Questions will likely involve one element forming a CCP or HCP lattice, and another element occupying a specific fraction (e.g., 1/3, 1/2, 2/3) of either tetrahedral or octahedral voids. Students need to be adept at calculating the number of voids (N octahedral, 2N tetrahedral for N atoms) and then deriving the simplest whole-number ratio for the compound's formula. This tests both conceptual understanding of voids and basic stoichiometric calculation skills.

Comparative Analysis of HCP vs. CCP/FCC

medium

Questions often aim to differentiate between HCP and CCP. This could be in the form of an 'incorrect statement' MCQ, or asking to identify the correct stacking sequence for a given structure, or listing metals that adopt a specific structure. The key is to remember the A-B-A-B vs. A-B-C-A-B-C stacking, the unit cell types (hexagonal vs. FCC), and common metallic examples. Understanding that both have CN=12 and PE=74% is also crucial for comparison.

Direct Recall of Packing Efficiency and Coordination Number

medium

Simple, direct recall questions on the packing efficiency values for different structures (SC, BCC, HCP, CCP) or the coordination number for 2D/3D close-packed structures are always possible. These are quick-scoring questions if the values are memorized. Sometimes, they might be embedded within a multi-concept question. For instance, 'Which structure has a packing efficiency of 74% and a coordination number of 12?'