Question

define valancy of an element

Answer 1

Valency is the combining capacity of an element

this is one of the best definition one can give u

Answer 2

the number of electrons present in the outer most shell of an element defines its valency. but if the number of outer shell is more than 8 than you need to subtract it from 8.

for example:-1) lithium-atomic number 3  but  2)nitrogen-atomic number-7
                    electric configuration- 2.1              electric configuration-2.5
              therefore valency is -1                        therefore valency is - 8-5=3