Nitrogen Anomaly

Question

Why does nitrogen not form pentahalides such as NCl_5 whereas phosphorus readily forms PCl_5 in the same group?

RankGuru · Accepted Answer

The ability to form pentahalides requires an element to expand its valence shell beyond an octet, which in turn requires accessible d-orbitals. Nitrogen belongs to the second period and has only 2s and 2p orbitals in its valence shell, with no 2d orbitals existing; therefore nitrogen cannot expand its octet and is limited to a maximum covalency of four, so NCl_5 cannot form. Phosphorus, in the third period, has empty 3d-orbitals available, allowing it to expand its octet and form PCl_5 with five bonds. The option that nitrogen is too electronegative is not the reason for restricted covalency. The option that phosphorus is smaller is factually wrong, since phosphorus is larger. The option of more valence electrons is incorrect, as both have five. This absence of d-orbitals in nitrogen is a key JEE Advanced anomaly from NCERT. Working through the logic step by step, rather than memorising the result, makes it clear why octet expansion governs the behaviour seen here. Plausibility check: nitrogen's maximum covalency of four, seen in the ammonium ion, and its inability to exceed an octet directly explain why NCl_5 does not exist, confirming the answer.

Nitrogen Anomaly

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