Voids in Close Packed Structures — Prelims Strategy

Prelims Strategy

To effectively tackle NEET questions on voids in close-packed structures, a systematic approach is essential.

1
Master the basics of close packing: — Clearly understand FCC (CCP) and HCP structures, including the effective number of atoms per unit cell (4 for FCC, 6 for HCP). This is the foundation for calculating voids.
2
Memorize void relationships: — Remember that for 'N' spheres in close packing, there are 'N' octahedral voids and '2N' tetrahedral voids. This 1:1:2 ratio is non-negotiable.
3
Visualize void locations: — For FCC, specifically know that octahedral voids are at the body center and edge centers, and tetrahedral voids are along the body diagonals (8 of them, 2 per diagonal). Practice drawing or visualizing these.
4
Practice stoichiometry problems: — These are very common. Always start by determining the effective number of the 'main' atoms (usually anions forming the lattice). Then, calculate the total number of relevant voids. Finally, apply the given occupancy fraction to find the number of the 'occupying' atoms (cations) and simplify the ratio to get the formula. Be careful with fractions.
5
Understand radius ratio: — Memorize the critical radius ratio values ($0.225$ for tetrahedral, $0.414$ for octahedral) and their corresponding ranges. This helps in predicting coordination numbers and void occupancy.
6
Avoid common traps: — Don't confuse the number of atoms in FCC vs. HCP. Double-check calculations, especially when dealing with fractions for void occupancy. Ensure you're using the correct void type (tetrahedral vs. octahedral) for the given problem. For conceptual questions, focus on precise definitions and locations.