Question

Why atoms have only 2 electrons in its first shell and how do We know it?

Answer 1

There are four quantum numbers :-
n,l,m,s. n is principal quantum number ,l is azimuthal quantum number , m is magnetic quantum number , s is spin quantum number.
Now, s is a subshell having value of l =0. So, magnetic quantum number gets a value 0. Magnetic quantum number tells us about orbitals. Now, since magnetic quantum number also has a value of 0,it suggests that there is only one orbital in s subshell.We know that every orbital can have maximum of two electrons(Pauli's exclusion principle).So, there are two electrons.
The Pauli exclusion principle is the quantum mechanical principle which states that two or more identical fermions (particles with half-integer spin) cannot occupy the same quantum state within a quantum system simultaneously.

Hope this helps :)