The formula of a noble gas species which is isostructural with \(BrO_3^-\) is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(XeO_3\)
Isostructural species have the same shape. Count the electron pairs round the central atom (VSEPR):
BrO3−: Br has 7 valence electrons, plus 1 for the negative charge = 8. Three are used for the three oxygen atoms (bond pairs) and the remaining electrons form one lone pair. 3 bond pairs + 1 lone pair: the pairs are tetrahedrally arranged and the shape is pyramidal.
XeO3: Xe has 8 valence electrons. Three Xe=O bonds and one lone pair on Xe: again 3 bond pairs + 1 lone pair, a pyramidal molecule.
Both also have the same number of valence electrons (\( \ce{BrO3^{-}} \): 7 + 3 × 6 + 1 = 26; \( \ce{XeO3} \): 8 + 3 × 6 = 26), so \( \ce{XeO3} \) is isostructural with \( \ce{BrO3^{-}} \).
Why the other options are incorrect
Option (1) XeOF4 — Incorrect. 5 bond pairs + 1 lone pair on Xe; the shape is square pyramidal.
Option (2) XeF2 — Incorrect. 2 bond pairs + 3 lone pairs on Xe; the shape is linear.
Option (4) XeF4 — Incorrect. 4 bond pairs + 2 lone pairs on Xe; the shape is square planar.
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.