Assertion: Although PF₅, PCl₅ and PBr₅ are known, the pentahalides of nitrogen have not been observed. Reason: Phosphorous has a lower electronegativity than nitrogen.

In phosphorous, due to the presence of vacant d-orbital, it is capable of extending its octet and thus forms pentahalides. However, in nitrogen, no d-orbital is present and thus it does not form pentahalides. Moving down the group, atomic size increases and electronegativity decreases due to lower net electron charge. Thus, nitrogen is more electronegative than phosphorous. 37 P-BLOCK ELEMENTS (GROUP 15-18) Therefore, the correct answer is an option (b) i.e. (B)both assertion and reason are true but the reason is not the correct explanation of assertion.