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With the help of atomic number we will find the electronic configuration then with the help of electronic configuration we will be ale to know the number of electron in last shell by which we can make out the valency of the electron.

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It is very easy and simple

1st point no of electrons in the last shell should be 8

use the formula 2n*n where n is the number of shells k,l,m etc

this formula is used to find the no of electrons that can be accommodated in a shell

for example if atomic number of an element is 34

now for first shell=2*1*1=2

for 2nd shell2*2*2=8

for third shell2*3*3=18

now by adding them we get 26 and

34-26=6

and we know that 8 electrons should be there in the lass shell 6-8=-2

hence valency is -2

1st point no of electrons in the last shell should be 8

use the formula 2n*n where n is the number of shells k,l,m etc

this formula is used to find the no of electrons that can be accommodated in a shell

for example if atomic number of an element is 34

now for first shell=2*1*1=2

for 2nd shell2*2*2=8

for third shell2*3*3=18

now by adding them we get 26 and

34-26=6

and we know that 8 electrons should be there in the lass shell 6-8=-2

hence valency is -2