Question

How the electron spin quantum number explains the line spectrum observed in case of multi electron atoms ?

Answer 1

The line-spectrum of multi-electron atoms consists of pairs of closely spaced lines. This means that there were twice as many energy levels as there should be. This can be explained only by assuming that electron can orient itself in two different spins having same energy. From here the concept of electron spin was introduced. This lead to the assignment of a fourth quantum number, the spin quantum number, s.The two possible values for electron spin being +1/2 and −1/2 which explains the pairing of line in line spectrum of multi-electron atom.