Question

The quantum numbers for an electron in the outermost orbital of an element are given as $n=2,l=0,m=0,s=-\frac{1}{2}$. The atom could be:

• A. Lithium
• B. Beryllium
• C. Hydrogen
• D. Boron

Answer 1

Answer: (A), (B)

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 $2s^1$ or $2s^2$. There could be another electron with the opposite spin the same orbital!
The electronic configuration can be $1s^{2}2s^1$ or $1s^{2}2s^2$ Hence, the element could be lithium (Z = 3) or Be (Z = 4)