The atom that contains only one electron in the highest occupied energy sublevel

Explanation:

The highest occupied energy sub-level is defined as number of electrons present in the outermost energy level of an atom.

For example, for alkali metals general electronic configuration is .

This means that in the highest occupied energy sub-level of an alkali metal there will be only one electron.

Whereas for alkaline earth metals general electronic configuration is . This means there are 2 electrons present in the highest occupied energy sub-level of an alkaline earth metal.

Thus, we can conclude that atom that contains only one electron in the highest occupied energy sublevel are alkali metals.