The problem states that the atom has two electrons in the outermost third energy level. This means n=3 is the highest principal quantum number with occupied electrons, and there are 2 electrons in t

The problem states that the atom has two electrons in the outermost third energy level. This means $n=3$ is the highest principal quantum number with occupied electrons, and there are 2 electrons in this level.

Within the third energy level, orbitals fill in the order $3s$, then $3p$, then $3d$. Since there are only two electrons in this level, they will occupy the $3s$ orbital.
So, the configuration for the outermost third energy level is $3s^2$.

For the third energy level to be the outermost occupied level, all preceding energy levels must be completely filled.
The first energy level ($n=1$) is filled with $1s^2$.
The second energy level ($n=2$) is filled with $2s^2 2p^6$.

Combining the configurations from Step 2 and Step 3:

This configuration corresponds to an atom with $2+2+6+2 = 12$ electrons, which is Magnesium (Mg).