Here's why:

* Radium (Ra) is an alkaline earth metal in Group 2 of the periodic table. It has a +2 charge when forming ionic compounds.

* Polonium (Po) is a metalloid in Group 16 of the periodic table. It has a -2 charge when forming ionic compounds.

Since the charges of radium and polonium are equal and opposite, they combine in a 1:1 ratio to form a neutral compound, RaPo.