In case of phosphorus, \(PCl_3\) and \(PCl_5\) are possible while nitrogen only forms \(NCl_3\), and not \(NCl_5\). This is due to:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Nitrogen does not have vacant d-orbitals in its atom while phosphorus has vacant d-orbitals.
Nitrogen (2s22p3) has only the n = 2 shell, with no d orbitals, so it can use at most four orbitals (one 2s and three 2p) for bonding; its covalency cannot exceed 4, and it cannot hold five bonds as in \( \ce{NCl5} \). Phosphorus (3s23p3) has vacant 3d orbitals; one 3s electron is promoted to 3d, giving five unpaired electrons and sp3d hybridisation, so it can form \( \ce{PCl5} \) as well as \( \ce{PCl3} \).
Why the other options are incorrect
Option (1) Nitrogen is a gas while phosphorus is a solid at room temperature. — Incorrect. The physical state of the element does not decide how many bonds its atom can form.
Option (3) The electronegativity of nitrogen is higher than that of phosphorus. — Incorrect. This is true but does not limit the number of bonds; the limit comes from the number of available orbitals.
Option (4) The atom of nitrogen is smaller in size than phosphorus. — Incorrect. Also true, and it favours pπ–pπ multiple bonding in nitrogen, but the absence of d orbitals is what stops N from expanding its covalency to five.
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):
Oxidation States and Trends in Chemical Reactivity – …
Chemical Properties – …
