A compound is formed by two elements \(P\) and \(Q\). The element \(Q\) forms ccp and atoms \(P\) occupy \(1/3 \)rd of tetrahedral voids. What is the formula of the compound?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → P2Q3
In a close-packed (ccp) arrangement of N atoms there are 2N tetrahedral voids (and N octahedral voids).
Let the number of Q atoms (forming the ccp lattice) = N.
Number of tetrahedral voids = 2N
Number of P atoms = \( \frac{1}{3} \times 2N = \frac{2N}{3} \)
P : Q = \( \frac{2N}{3} : N = 2 : 3 \)
So the formula of the compound is \( \ce{P2Q3} \).
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.
NCERT (Pre-rationalised edition, 2022-23):
Placing Second Layer over the First Layer – “… Let the number of close packed spheres be N, then:
The number of octahedral voids generated = N
The number of tetrahedral voids generated = 2N”