CBSE Class 9 Answered

Valency refers to the configuration of the outer electron of an atom, and give its potential to form chemical bonds in terms of the number of electrons it is able to share to surrender or gain. A single atom can have multiple valencies: that is may be able, in the case of nitrogen for example, to lose 3 or gain 5 electrons, and then it would have multiple valencies: 3 and 5.

The no. of electrons in last shell should be 8
 Also, remember to use 2n*n where n is the no. of the shell K,L,M etc. this formula is used to find no. of electrons that can be accommodated in a shell
For example, if you want valency of element with atomic no.34
Then Using the formula
For 1st shell 2*1*1=2
For 2nd shell 2*2*2=8
For 3rd shell 2*3*3=18
Here's the trick adding the electrons in these shells.=28 And the no. left is 6
So as we know we need 8 electrons in last shell so, 2 electrons are less so the valency is -2
if the atomic no.were 36 it would be a noble gas valency=0, 37 then a valency =+1