Question

define valency by taking examples of magnesium (At No=12) and oxygen (At No=8)​

Answer 1

In chemistry, the valence or valency of an element is a measure of its combining power with other atoms when it forms chemical compounds or molecules. The concept of valence developed in the second half of the 19th century and helped successfully explain the molecular structure of inorganic and organic compounds. 

magnesium has atomic number 12 show the electronic configuration is 2,8 and 2 the valency will be 2 because the valence electron is 2

ox has the atomic number of 8 show the electronic configuration is 2,6

Answer 2

Valency is the combining capicity of an atom.

For eg : Mg

K. L. M.
2. 8. 2

Now to find valency :
If outer shell contains 1,2,3,4 then valency will be same as electrons in outer shell.

But it is more then 4 then :

8 - No. of electrons in outer shell

so the valency of Mg will be 2.

Oxygen : 8 - 6 = 2