Explain why the quantum number set (2, 1, - 2, - ½) is not possible for an electron in an atom.

Because electrons in an atom can occupy only certain definite positions with respect to the nucleus. These are known as stationary states. These stationary states are also called energy levels. therefore it have characterized in the form of interger (1,2,3,4 ...) from lowest level to highest level. or we can say in term of series. therefore it is not possible in (2,1,-2, ...) since it occur in a series form.