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.

"the periodic table is arranged in order of atomic number."