please expain about spin quantum number...

In an orbital there can accomodate 2 electrons, but according to pauli exclusion principle- no 2 electrons in an atom can have the same set of 4 quantum nos, so the 2 electrons in the same orbital stay together by having opposite spins. this opposite spin is shown by the spin quantum no.

It designates the orientation of the spin of an electron. that is spin quantum no. tells that the electron revolve in clockwise or anti-clockwise direction. For electron in clockwise direction it is given +1/2 and for electron in anti-clockwise direction it is given -1/2.