Close Packed Structures — Prelims Strategy

Prelims Strategy

To effectively tackle NEET questions on close-packed structures, a multi-pronged strategy is essential:

1
Conceptual Clarity: — Ensure a crystal-clear understanding of 1D, 2D (square vs. hexagonal), and 3D (HCP vs. CCP) packing. Focus on how each is formed and its key characteristics.
2
Memorize Key Facts: — Commit to memory the coordination numbers for different packing types (2, 4, 6, 8, 12), packing efficiencies (SC: 52.4%, BCC: 68%, HCP/CCP: 74%), and the stacking sequences (HCP: A-B-A-B, CCP: A-B-C-A-B-C). Also, remember common examples of metals for HCP (Mg, Zn) and CCP (Cu, Ag, Au, Al).
3
Void Relationships: — This is critical. Always remember that for 'N' atoms in a close-packed structure, there are 'N' octahedral voids and '2N' tetrahedral voids. This ratio is the backbone for stoichiometry problems.
4
Numerical Problem Solving: — For questions involving void occupancy and compound formulas, follow a systematic approach:

* Identify the type of lattice formed by the larger atoms (usually anions). This gives you 'N' (e.g., N=4 for FCC/CCP). * Calculate the total number of tetrahedral (2N) and octahedral (N) voids. * Determine the number of smaller atoms (cations) based on the given fraction of occupied voids. * Establish the simplest whole-number ratio between the larger and smaller atoms to find the empirical formula.

1
Trap Options: — Be wary of options that swap the number of tetrahedral and octahedral voids, or use incorrect packing efficiencies. Always double-check the question for keywords like '2D', '3D', 'tetrahedral', 'octahedral', 'HCP', 'CCP'. Practice with previous year questions to identify common traps.