Question

definition of valency is combining capacity of an element is called valency. but, actual meaning of valency is the electrons present in the outer shell is called valency? how do they match it????????????????????????????????????????????

Answer 1

total number of electrons present in last shell

Answer 2

Good Question !!
The valency of an element can be defined in the two ways listed :
a) Combining capacity of an element
b) Electrons present in the outermost shell

These both definitions are related
Let me tell you with the help of an example

Let us take Sodium(Na)
Its Electronic Configuration : 2,8,1
Here we see that it has 1 electron in its valence shell.So it wants to get rid of that 1 electron to become stable.
It can be achieved in 2 ways :
> Accepting 7 electrons
> Donating 1 electron

Donating 1 electron is easier so we can say that each sodium atom has a valency of +1 i.e each sodium atom can give away one electron to gain stability.

Let us take one more example of chlorine(Cl)

Electronic Configuration : 2,8,7
Chlorine has 7 electron in its valence shell so stability can be achieved in 2 ways:
> Losing 7 electrons
> Gaining 1 electron

Accepting 1 electron is easier so chlorine has a valency of -1 i.e Chlorine can take 1 electron

The valency of first 20 elements show a good pattern to be memorized:
H = +1
He = 0
Li = +1
Be = +2
B = +3
C = 4
N = -3
O = -2
Fl = -1
Ne = 0
Na = +1
Mg = +2
Al = +3
Si = 4
P = -3
S = -2
Cl = -1
Ar = 0
K = +1
Ca = +2

Hope you like the answer ;)