Chemistry Chapter 2: Structure Of Atom | Part - 4 | Valency And Electronic Configuration

Valence Electron :
As we have learned that electrons in an atom are arranged in different orbits or shells. 
The electrons that are present in the outermost orbit or shell of an atom are known as the valence electron.
They determine the valency of an atom.

Valency :
From the Bohr-Bury scheme, we acknowledged that the outermost shell of an atom can accommodate a maximum of 8 electrons.
It has been found at if the outermost shell of an atom is completely filled with 8 electrons then, their combining capacity or valency will be zero. 
They will be known as inert elements which can’t react with any other elements whereas in the case of helium, they contain two electrons in their outermost shell.
Valency can be described as the combining capacity of an atom or the tendency to react and form molecules with the atom of the same or different elements.
It is basically done to attain a completely filled outermost shell i.e., eight electrons or octet.
This is done by sharing, gaining, or losing electrons just to make the octet of electrons in the outermost shell.

How to find valency of an atom :
The helium atom has 2 electrons in its outermost orbit thus, its valency would be also 2. 
It can also be described as helium sharing 2 electrons to form a chemical bond with any other element.
Example – 
If an atom has 3 electrons in its outermost orbit ( valence electrons= 3 ) then its valency would be 3.
Here, losing of valence electrons is taking place.
Valences are basically the least number of electrons that are gained, lost, or shared so as to make the duplet or octet of electrons in the outermost shell.
Boron (B) = Atomic no. is 5.
             K = 2L = 3 
So, to gain the atomic configuration of helium, there is a need to lose the electrons of orbit L.
Thus, here valence electron is 3 and valency is 3 too.
Example –
If an atom has 6 as a valence electron then, to complete octet it would be easier to gain 2 more electrons so as to make it 8electrons or octet. 
             6 + 2 = 8 
             Thus, its valency will be 2.
Example –Nitrogen (N) – Atomic no. = 7
             K = 2,  L = 5 
To complete its octet, there is need to gain 3 electrons more, so that–
             K = 2,  L = 5 + 3 = 8 
Here, valency would be 3 
Example – Silicon (Si) – Atomic no. = 14
             K = 2, L = 8, M = 4
To complete its octet, there is a need to share or gain 4 electrons.
Now, K = 2 L = 8M = 4 + 4 = 8 Thus, its valency is 4.

Electronic configuration of first eighteen elements