The quantum numbers for an electron in the outermost orbital of an element are given as n=2,l=0,m=0,s=−12. The atom could be:
Lithium
Beryllium
This is talking about the electron belonging to the 2s subshell. On reading the question carefully, you will see that the outermost orbital is being spoken about, NOT the outermost electron. This means we could be talking about 2s1 or 2s2. There could be another electron with the opposite spin the same orbital!
The electronic configuration can be 1s2 2s1 or 1s2 2s2 Hence, the element could be lithium (Z = 3) or Be (Z = 4)