When two different halogens react with each other, interhalogen compounds are formed. They can be assigned general compositions as \(XX'\), \(XX'_3\), \(XX'_5\) and \(XX'_7\), where \(X\) is a halogen of larger size and \(X'\) of smaller size and \(X\) is more electropositive than \(X'\). As the ratio between radii of \(X\) and \(X'\) increases, the number of atoms per molecule also increases.
On the basis of above passage, answer the questions
In interhalogens \(XX'_7\), what are \(X\) and \(X'\) respectively?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Iodine and fluorine
In interhalogens XX′n, X is the halogen of larger size (more electropositive) and X′ the smaller one. As the ratio between the radii of X and X′ increases, more X′ atoms can fit around X, so the number of atoms per molecule increases.
The ratio of radii is largest for iodine (largest halogen) and fluorine (smallest halogen), so iodine(VII) fluoride has the maximum number of atoms: its formula is IF7, the only XX′7 interhalogen. Its shape is pentagonal bipyramidal.
Why the other options are incorrect
Option (1) Chlorine and fluorine — Incorrect. Chlorine is too small to hold seven fluorine atoms; the highest chlorine fluoride is \( \ce{ClF5} \).
Option (3) Bromine and fluorine — Incorrect. The highest bromine fluoride is \( \ce{BrF5} \); the Br/F size ratio is not large enough for \( \ce{BrF7} \).
Option (4) Chlorine and bromine — Incorrect. These two are close in size; they form only BrCl (XX′ type).
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):
Interhalogen Compounds
