Question

Which one of the compounds is not known?

• A. ${\mathrm{NCl}}_5$
• B. ${\mathrm{PCl}}_5$
• C. ${\mathrm{AsCl}}_5$
• D. ${\mathrm{NCl}}_3$

Answer 1

The correct option is A

${\mathrm{NCl}}_5$

Explanation for the correct option:

(A) ${\mathrm{NCl}}_5$

• Nitrogen is an element with an atomic number $7$, the valency of Nitrogen is $3$.
• Nitrogen does not have d orbitals to expand its covalence beyond three.
• The d-orbitals are absent in N, so electrons can only be accommodated in the p-orbitals of $5$ $\mathrm{Cl}$atoms. As a result,${\mathrm{NCl}}_5$ does not exist.

Explanation for incorrect options:

(B) ${\mathrm{PCl}}_5$

• The atomic number of Phosphorus is $15$, it contains empty d orbitals, thus it can accept electrons from Chlorine atoms.
• Thus, ${\mathrm{PCl}}_5$ exist.

(C) ${\mathrm{AsCl}}_5$

• The atomic number of Arsenic is $33$, it contains empty d orbitals, thus it can accept electrons from Chlorine atoms.
• Thus, ${\mathrm{AsCl}}_5$ exist.

(D) ${\mathrm{NCl}}_3$

• Nitrogen is an element with an atomic number $7$, the valency of Nitrogen is $3$.
• It can accommodate three chlorine atoms easily. Thus, ${\mathrm{NCl}}_3$ exist.

Hence, the correct option is A i.e. ${\mathrm{NCl}}_5$does not exist.