An alloy of Cu, Ag and Au is found to have Cu constituting the CCP lattice. If Ag atoms occupy edge centres, and Au is present at body centre, then the alloy has formula
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(Cu_4Ag_3Au\)
The formula of a unit cell is found by counting how much of each atom belongs to one cell. A corner atom is shared by 8 cells (1/8 each), a face centre by 2 (1/2), an edge centre by 4 (1/4), and a body centre belongs to one cell only.
\( \displaystyle \text{Cu (ccp):}\ 8 \times \frac{1}{8} + 6 \times \frac{1}{2} = 1 + 3 = 4 \)
\( \displaystyle \text{Ag (12 edge centres):}\ 12 \times \frac{1}{4} = 3 \)
\( \displaystyle \text{Au (body centre):}\ 1 \times 1 = 1 \)
So Cu : Ag : Au = 4 : 3 : 1 and the formula is Cu4Ag3Au.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.