Here's why:

* Alkaline earth metals are a group of elements in the second column of the periodic table.

* They are characterized by having two valence electrons, making them highly reactive.

* Radium is the heaviest naturally occurring element in this group, with an atomic number of 88.

While there are synthetically created elements with higher atomic numbers that could potentially be considered alkaline earth metals, they are not naturally occurring and are extremely unstable.