# 1. Why is Frenkel defect not found in alkali metal halides? 2. A crystalline solid has simple cubic structure in which

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2. A crystalline solid has simple cubic structure in which P atoms are present at the corners, Q atoms are present at the edge centres and R atoms are present at the centre of the unit cell. What is the formula of the compound?

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1. Frenkel defect is favoured by the small size of cations. In alkali metal halides both cations and anions are of almost same size.

2. Number of atoms of P = $\frac{1}{8}$ × 8 = 1

Number of atoms of Q = $\frac{1}{4}$ × 12 = 3

Number of atoms of R = 1

∴ Formula of the compound is PQ3R