Atoms of element B form hcp lattice and those of the element A occupy \(\frac{2}{3}^{rd}\) of octahedral voids. What is the formula of the compound formed by the element A and B?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A4B3
In an hcp lattice of N atoms of B there are N octahedral voids and 2N tetrahedral voids. With A in 2/3 of the tetrahedral voids:
\( \displaystyle \text{Number of A} = \frac{2}{3} \times 2N = \frac{4N}{3} \)
\( \displaystyle A : B = \frac{4N}{3} : N = 4 : 3 \;\Rightarrow\; A_4B_3 \)
Check: with A in 2/3 of the octahedral voids, A = 2N/3 and A : B = 2 : 3 (A2B3), which is not among the options.
Note 1: It seems that there is a mistake in the original NTA paper, which prints 2/3 of the octahedral voids when it should read 2/3 of the tetrahedral voids.
Note 2: 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
