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

Comparing (2, 2, - 1, - ½) with (n, l, m, s) we have

n = principal quantum number = 2

l = azimuthal quantum number = 2

m = magnetic quantum number = - 1

s = spin quantum number = - ½

Now, for every 'n', the 'l' values are having only l = n-1 integers. But on looking above, we see that 'l' is having value 2 which is same for 'n' which violates the above concept. The rest all quantum numbers are correct. Hence, the quantum number set (2, 2, - 1, - ½) is not possible for an electron in an atom.