Question

Which of the compounds is not known?
(A) $ NC{l_5} $
(B) $ PC{l_5} $
(C) $ AsC{l_5} $
(D) $ NC{l_3} $

Answer 1

Hint :The given elements are the halides of Group $ 15 $ of the periodic table. Group $ 15 $ mostly has a valency of three. Hence, we’ll answer this question according to the characteristics of Group $ 15 $ .

Complete Step By Step Answer:
The first compound given to us is $ NC{l_5} $ . The electronic configuration of nitrogen is $ 1{s^2}2{s^2}2{p^3} $ . Nitrogen belongs to the second period of the periodic table; hence nitrogen doesn’t have d-orbitals present. The valency of nitrogen is Three, and it can form only three bonds.
The given compound $ NC{l_5} $ has five Nitrogen Chlorine bonds. Nitrogen cannot extend its octet due to the absence of the d-orbital. Hence nitrogen cannot form five bonds, it can only form three bonds. Therefore, the compound $ NC{l_5} $ is not known.
Phosphorus belongs to the third period of the periodic table and has d-orbitals present in it. Hence phosphorus can extend its octet and form more than three bonds also. The compound $ PC{l_5} $ exists via forming dative bonds and Option 2) is incorrect.
Arsenic being below phosphorus can also extend its octet and form five bonds, hence option 3) is also incorrect.
Nitrogen cannot form five bonds, but can surely form three bonds, hence option 4) is also incorrect as $ NC{l_3} $ exists in reality.
Option (1) is the correct answer.

Note :
This inability of nitrogen to extend its octet and form only three bonds comes under the anomalous behavior of nitrogen. Other anomalous behavior includes the presence of nitrogen as diatomic whereas others as polyatomic, High electronegativity and ionization energy of Nitrogen, non-availability of valence d-shell orbital, and its small size.