Question

With the help of Pauli's exclusion principle show that there can be a maximum of two electrons in an orbital?

Answer 1

It says that no two electrons can have same set of 4 quantum number

It means each orbital can accomodate maximum 2 electrons of opposite spin

Example- Li = 3 = 1s^3

n=principal quantum no. l=azimuthal quantum no.

ml=magnetic quantum no. ms=spin quantum no.

Now

1st electron- n=1 l=0 ml=0 ms= +1/2

2nd electron- n=1 l=0 ml=0 ms= -1/2

3rd electron- n=1 l=0 ml=0 ms= +1/2

Here the quantum number set of third electron is same as that of first electron which is not valid. hence the third electron is excluded from 1s and shifted to next orbital which is 2s

correct electronic configuration of Li 1s^2 2s^1