Question

Penta-valence in phosphorus is more stable when compared to that of nitrogen even though they belong to same group is due to
(A) Reactivity of phosphorus
(B) Inert nature of nitrogen
(C) Hydrogen and sulphurous acid
(D) Large size of phosphorus atom

Answer 1

Hint: Phosphorus and nitrogen belong to the same group but different periods. Nitrogen has valence shell electronic configuration $2{s^2}2{p^3}$. Whereas phosphorus has electronic configuration of the valence shell $3{s^2}3{p^3}3{d^0}$.

Complete Step by Step Solution:
phosphorus belongs to the third period of the periodic table. Hence its valence shell has $3s,3p,3d$. But nitrogen belongs to the second period of the periodic table thus, having valence shell $2s$and $2p$. We all know that $2d$ does not exist because for a given subshell, the value of azimuthal quantum number cannot be equal or greater than the principal quantum number.

Thus, in phosphorus, the pair of electrons present in $3s$can unpair itself and a single electron can transfer to a $3d$ subshell, making the number of unpaired electrons $5$. Hence the maximum covalency of phosphorus is $5$. This was possible due to the low energy gap between $3s$and $3d$.

But nitrogen is not pentavalent because of unavailability of low-lying vacant orbital. $2d$orbital is not present in nitrogen to cause excitation of $4s$electrons and a very high amount of energy is required to excite them to other vacant orbitals of the third shell.
Hence, phosphorus is pentavalent due to the large size of phosphorus.
The correct answer is D.

Note: Increase in covalency is due to excitation of electrons. This excitation of electrons can be free as well as forced. $2nd$ period elements do not show excitation because of the absence of low-lying vacant orbitals. Also, expansion of covalency is only possible in $3rd$ period elements when there is charge contraction due to presence of a side atom with greater electronegativity along with low- lying vacant orbitals.