Quantum numbers were earlier postulated in Bohr-Somerfield theory to describe the various orbits. Since quantum number were not actually derived form the theory itself but proposed out of necessity, therefore, this was considered as a fundamental weakness of the Bohr atom. On the other hand, the three quantum number n,and m and their allowed values arise as a consequence of the solution of the wave equation for hydrogen atom.

In wave mechanics, the electron distribution in an atom is divided into shells which in turn are supposed to of one or more subshells and the subshells  are assumed to be made up of one or more orbital’s. The electrons occupy these orbital’s. “ Each electron in an atom is identified in terms of four quantum numbers out of which n, and m arise form the wave equation and the fourth, spin quantum number, was proposed out of necessity. The quantum number in this order indicate, somewhat loosely’ shell, subshell, orbital and electron.

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