Packing in Solids — Prelims Strategy

Prelims Strategy

To excel in 'Packing in Solids' questions for NEET, a systematic approach is vital. Firstly, memorize key values: packing efficiencies (SC: 52.4%, BCC: 68%, FCC/HCP: 74%), coordination numbers (SC: 6, BCC: 8, FCC/HCP: 12), and the number of atoms per unit cell (Z) for cubic systems (SC: 1, BCC: 2, FCC: 4). These are direct recall facts often tested.

Secondly, understand the relationships between 'a' and 'r' for each cubic unit cell: $a=2r$ for SC, $a=4r/sqrt{3}$ for BCC, and $a=2sqrt{2}r$ for FCC. Practice deriving these relationships to solidify your understanding, as numerical problems frequently involve these. When solving numerical problems, always write down the given values and the formula clearly. Pay attention to units (pm, nm, Å).

Thirdly, master voids. Remember that for 'N' atoms in a close-packed structure, there are '2N' tetrahedral voids and 'N' octahedral voids. Understand their locations within the FCC unit cell (tetrahedral voids along body diagonals, octahedral voids at body center and edge centers). Questions on compound formulas based on void occupancy are common; practice setting up ratios of atoms based on their positions and void filling.

Finally, visualize the structures. Use diagrams or 3D models if available. This helps in understanding stacking patterns (ABA vs. ABC for HCP vs. CCP) and the concept of nearest neighbors for coordination number. Be careful with trap options that swap values between different lattice types or use incorrect 'a' vs 'r' relationships. Always double-check your calculations and conceptual reasoning.