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.
