Atomic number $\left(15\right)$ electronic configuration is
$\left(15\right)\to 1{\mathrm{s}}^2,2{\mathrm{s}}^2,2{\mathrm{p}}^6,3{\mathrm{s}}^2,3{\mathrm{p}}^3$
and this element is Phosphorous.
Group no $=15$
Valence ${\mathrm{e}}^{-}\Rightarrow {\mathrm{e}}^{-}$ present in valence shell
$=2+3=5$
Valency $=3$ , it needs $3$ electron to complete its octet.