Solution: Radium is a metal and oxygen is a nonmetal, making the compound ionic. To write the formula for ionic compounds, we first figure out the oxidation numbers and then cross multiply them. Radium is in Group 2 on the Periodic Table and has an oxidation number of +2. Oxygen is in Group 16 and has an oxidation number of -2. Since both number are the same, we need one radium for one oxygen to get a neutral molecule. We get RaO, choice a.
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