Packing Efficiency — Prelims Strategy

Prelims Strategy

To excel in NEET questions on packing efficiency, a multi-pronged strategy is essential. First, memorize the packing efficiencies for the three main cubic unit cells: Simple Cubic (SC) is $52.4\%$, Body-Centered Cubic (BCC) is $68\%$, and Face-Centered Cubic (FCC) is $74\%$.

Also, remember the corresponding void spaces (100% - PE). Second, understand and memorize the relationship between the atomic radius (r) and the edge length (a) for each structure: $a=2r$ for SC, $a=\frac{4r}{\sqrt{3}}$ for BCC, and $a=2\sqrt{2}r$ for FCC.

These are frequently tested. Third, practice numerical problems extensively. Be comfortable calculating 'a' from 'r', 'r' from 'a', or even the volume of the unit cell or occupied spheres. For conceptual questions, focus on comparing the efficiencies and understanding their implications for properties like density.

Avoid common traps like confusing packing efficiency with void space or mixing up the 'a-r' relationships between different unit cells. Always double-check the unit cell type mentioned in the question before applying any formula.