Question

Phosphorus form pcl5 but nitrogen doesn't form ncl5 why ?

Answer 1

Nitrogen has no free d-orbital in its valence cell electron so Phosphorus can form pcl5 but nitrogen doesn't form ncl5.

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Answer 2

Nitrogen being the first element of group 15 is the smallest.Being very small it has a high electronegativity and no vacant d orbitals available for excitation of electrons.Thus NCl5 does not exist but PCl5 exists.