Which of the following represents the lattice structure of A_(0.95)O containing A²⁺,A³⁺ and O²⁻ ions?

Solution

Chemistry ma’am · 3 steps

Look for a missing cation: each empty A²⁺ site is paid for by two A³⁺ ions nearby.

1. Read the formula. A_(0.95)O has fewer cations than oxide ions, like Fe_(0.95)O. This is a metal-deficiency defect: some A²⁺ sites are empty, and the crystal must still be neutral.
2. Keep it neutral. One missing A²⁺ removes a charge of +2. Two neighbouring A²⁺ ions become A³⁺ to give back exactly +2. So the correct lattice has one cation vacancy, two A³⁺ on cation sites, and every oxide ion in its normal place.

   Lattice	What you see	Cations : O²⁻	Total charge
   A	one empty A²⁺ site, two A³⁺ on cation sites	19 : 20 = 0.95	34 + 6 = +40 vs -40, neutral
   B	two A³⁺ sitting on oxide-ion sites, no vacancy	20 : 16, metal excess	+42 vs -32, not neutral
   C	one A³⁺, two extra O²⁻ in interstitial spaces	25 : 26	+51 vs -52, not neutral

3. So. Only lattice A has the formula A_(0.95)O and stays electrically neutral, so the answer is (c), A only.

The idea to keep. In metal-deficient oxides like Fe_(0.95)O, a cation is missing and two nearby cations take a higher charge. Always count the charges: the lattice must come out neutral.

Answer from the NTA final answer key. Solution by PrepWiser.