What is the valency of an element with the electronic configuration 2, 8, 7?

This element has 7 electrons in its outer shell. It needs one more electron to complete it. So valency is 1.

Elements with 7 valence electrons gain one electron. They form -1 ions. This makes them Group 17 halogens.

Remember: Valency shows how many electrons an atom gains, loses, or shares. Seven electrons means gain one.