$word_{count}$: 200

Solid state contributes ~3% of JEE Main marks. Priority areas: (1) Density calculation — d = $\frac{ZM}{N_Aa^3}$. Know Z for SC(1), BCC(2), FCC(4). Convert pm to cm. (2) Packing efficiency — derive or memorise: 52.4%, 68%, 74%. (3) Voids — tetrahedral = 2n, octahedral = n. Know which voids are filled in NaCl (all oct), ZnS (half tet), CaF2 (all tet). (4) Defects — Schottky vs Frenkel: density change, examples, conditions. (5) CsCl is NOT BCC — this appears as a trap MCQ almost every year. Common mistakes: (a) Forgetting to convert pm to cm in density formula. (b) Confusing BCC with CsCl structure. (c) Using wrong Z value. (d) Forgetting that packing efficiency of HCP = FCC. (e) Confusing octahedral void count (n) with tetrahedral (2n). (f) Saying Frenkel defect decreases density (it doesn't — atoms don't leave). Mental math: memorise r/a relationships and packing percentages. The density formula can be rearranged to solve for any unknown — practise all four rearrangements.