Here's why:

* Alkaline-earth metals are a group of elements in the second column of the periodic table. They are known for their reactivity and metallic properties.

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

* Atomic number determines the number of protons in an atom's nucleus. The higher the atomic number, the heavier the element.

While radium is the heaviest, it's also incredibly radioactive and very rare.