Answer :
No of protons=No of electrons=15
Mass no:-No of neutrons+No of protons
[tex]\\ \tt\longmapsto 15+16=31[/tex]
Now
It has atomic no 15 hence its Phosphorus(P)
Electronic configuration:-
[tex]\\ \tt\longmapsto 1s^22s^22p^63s^23p^3[/tex].
- Valency=3
Chloride formula:-
[tex]\\ \tt\longmapsto XCl_3[/tex]
or.
[tex]\\ \tt\longmapsto PCl_3[/tex]