Question

how to calculate valency no.

Answer 1

The valency of an element is defined by the number of electrons in its valence shell.The valency of an atom is equal to the number of electrons in the outer shell if its four or less than four. If the number of valence electron is greater than four then valency can be found out by 8 minus the number of valence electron.

Answer 2

by calculating no. of electron in 
outermost shell of the element