If Z = 5, what would be the valency of the element?

Z denoted the atomic number of the element.

Therefore the element is Boron because atomic number of boron is 5.

Number of electrons Present in atom: 5;

Electronic configuration of atom: 2, 3.

Valency: since Boron has 3 electrons in its outer most shell .Hence it can give the three electrons in order to complete its octet. Therefore the valency of element is 3.