An alkaline earth metal in period $3$ is?

Alkaline earth metal

• The general electronic configuration for alkaline earth metals is $\left[\text{noble gas}\right]n{s}^2$.
• The alkaline earth metals are beryllium, magnesium, calcium, strontium, barium, and radium in the periodic table.

Alkaline earth metal in period $3$

• Alkaline earth metal in period $3$ is magnesium.
• The symbol for magnesium is $\mathrm{Mg}$.

So, an alkaline earth metal in period $3$ is magnesium.