Question

how to learn vallency and atomic mass in chemustry give its shortest way to learn

Answer 1

First, understand correctly what valency means: Valency is dependent on the number of valence electrons. Valence electrons are the electrons present in the outermost shell of an atom.
For an atom to be stable, it needs to complete its octet, meaning it has to have eight electrons in outermost shell.
Having remembered this, you can now easily learn to find valency by knowing its atomic number.
Suppose the atomic number is 10. So now according to formula
$2n^{2}$
You get the number of electrons in each shell. In this case, electrons are distributed: 2,8. Thus, outermost shell has 8 electrons. So it needs 0 electrons to complete the octet.
So the easiest way is to
1) Find its K, L, M, N electronic configuration from atomic number
2) Subtract the number of electrons in outermost shell from 8 to get valency.
Remember, sometimes valency can be negative or positive.

As to atomic mass, you just have to learn them mechanically. It can be approximated that atomic mass is usually close to the double of atomic number.

Hope this helps!