UPSC CDS 2 2020 GK Paper

Concept:Valency is the combining capacity of an atom, measured by the number of electrons it gains, loses, or shares to achieve a stable octet.Explanation:The atomic number of phosphorus is $15$.Its ground‑state electronic configuration is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{3}$, or $[\mathrm{Ne}]3s^{2}3p^{3}$.This shows five valence electrons in the $3s$ and $3p$ subshells.To reach a stable octet (like argon), phosphorus can gain three electrons, giving a valency of $3$.Alternatively, it can lose all five valence electrons, showing a valency of $5$.Phosphorus also has vacant $3d$ orbitals.In an excited state, one electron from $3s$ is promoted to $3d$, giving $3s^{1}3p^{3}3d^{1}$.Now there are five unpaired electrons, allowing phosphorus to form five bonds (valency $5$).Hence, phosphorus shows variable valency: $3$ and $5$.Answer:Option D ($3,5$)