Class 12 Chemistry Chapter 1 Notes - Solid State

The most important topics for JEE Main are packing efficiency, number of atoms per unit cell, density calculations using the unit cell formula, and types of crystal defects (Schottky and Frenkel). NTA's official question papers from 2019–2024 show that packing efficiency and defects together account for nearly 60% of questions from this chapter.

Schottky defects involve equal numbers of cations and anions missing from lattice sites — density decreases and it occurs in solids with similar-sized ions like NaCl. Frenkel defects involve a cation shifting to an interstitial position — density stays nearly the same and it occurs in solids with large size differences between ions, like AgCl and ZnS.

In an FCC unit cell, there are 4 atoms. The face diagonal equals 4r, so the edge length a = 4r/√2 = 2√2 r. Volume of 4 atoms = 4 × (4/3)πr³. Volume of unit cell = a³ = (2√2 r)³ = 16√2 r³. Packing efficiency = (4 × 4πr³/3) / (16√2 r³) × 100 = 74.05%.

An F-centre (Farbe centre, from the German word for colour) is an anionic vacancy in a crystal that is occupied by an electron. These trapped electrons absorb visible light and cause the crystal to appear coloured. Excess sodium in NaCl creates F-centres that make the crystal appear yellow. This concept appears in both JEE and NEET.

In a BCC (body-centred cubic) structure, the coordination number is 8 — each atom is directly surrounded by 8 nearest neighbours. In an FCC (face-centred cubic) structure, the coordination number is 12 — each atom is surrounded by 12 nearest neighbours. Higher coordination number means more efficient packing and greater density.