Question

What are the methods of finding valency?

Answer 1

HI !

Valency is known as the combining capacity of elements .

We can find the valency of an element by , with the help of valence electrons , by doing the electronic configuration of the element.

Electrons surround the atom , in shells.
The shells are named K,L,M,N......
No: of electrons in each shell is given by 2n² , where n is the shell no: ( n = 1,2,3....)

K - shell no: 1
L - shell no:2
etc.....

No: of electrons in each shell :-

K shell = 2n² = 2*1² = 2 electrons ( maximum) 
L shell = 2n² = 2*2² = 8 electrons ( max.)
M shell  = 2n² = 2*3² = 18 

As the no: of electrons in the outermost shell must be 8 electrons , M shell( if it is the outermost shell of an atom , will have 8 electrons only)

Now ,let do the electronic configuration of an element ( Sodium - Na)

We know that ,

Atomic no: of sodium = Z = 11
No: of protons = No: of electrons = Z = 11

Electronic configuration = 2,8,1
k shell = 2
l shell = 8
m shell = 1

Valence electrons is the no: of electrons present in outermost shell.
If valence electrons is less than 5 , the valency = no: of electrons in outermost shell.
If valence electrons is greater than 5 , the valency = 8 - no: of electrons in outermost shell.

Here , valence electron = 1

1 < 5 

Hence,

Valency of sodium = 1

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Another eg: ,

Oxygen (O)

Atomic no: = 8

 Atomic no: of sodium = Z = 8
No: of protons = No: of electrons = Z = 8

Electronic configuration = 2,6
k shell = 2
l shell = 6

valence electron = 6 ( greater than 5)

hence,

valency = 8 - valence electron
             = 8 - 6
             =2

valency of oxygen = 2