Find the valency of the elements with these atomic numbers: 6, 9, 11, 13 and 15.

Valency is the number of electrons an atom loses, gains or shares to fill its outer shell. With $1$ to $4$ outer electrons it equals that number; with $5$ to $7$ it is $8$ minus that number; a full shell gives $0$. Cross the valencies to write a formula.