Question

An element has an electronic configuration of 2 8 7. Its valence is

Answer 1

the element is chlorine
it's valency is 1

Answer 2

Valency is 1 .

The last number of the electronic configuration can tell us about the valency of the element.

In your question the last shell of the element contains 7 electrons. So it requires only 1 electron to become stable and attain the nearby noble gas configuration. Hence it's valency becomes one.

All in all, just remember that valency is the no. of electrons that an atom of an element takes up or loses to become stable..