A compound is formed by 2 elements P and Q. Atoms of element Q (as anions) make ccp and those of P (as cations) fill all tetrahedral voids and also occupy half of the octahedral voids. What is the correct formula of this compound?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \(P_5Q_2\)
Let the number of Q atoms in the ccp lattice be N. Then
Octahedral voids = N Tetrahedral voids = 2N
P fills all tetrahedral voids: 2N
P fills half the octahedral voids: N/2
Total P = 2N + N/2 = 5N/2
P : Q = 5N/2 : N = 5 : 2, so the formula is P5Q2.
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):
Formula of a Compound and Number of Voids Filled – “… Let the number of close packed spheres be N, then:
The number of octahedral voids generated = N
The number of tetrahedral voids generated = 2N”