Answer
(a) cobalt (II) oxide
(b) cobalt (III) oxide
(c) cobalt (IV) oxide
Work Step by Step
Cobalt (Co), a transition metal, has several oxidation numbers. To specify which oxidation number this particular atom of cobalt has, we use Roman numerals inside parentheses and place them following the name cobalt. We then add the name of the anion, changing its ending to $-ide$.
(a) In this compound of cobalt, we see that we need one atom of oxygen (O) for every atom of cobalt. If oxygen has an oxidation number of $2-$, then we know this atom of cobalt has an oxidation number of $2+$. We represent this oxidation number with the Roman numeral II in parentheses following the name of the cation, cobalt. Therefore, the name of this compound is cobalt (II) oxide.
(b) In this compound of cobalt, we see that we need three atoms of oxygen (O) for every two atoms of cobalt. If oxygen has an oxidation number of $2-$, then we know this atom of cobalt has an oxidation number of $3+$. We represent this oxidation number with the Roman numeral III in parentheses following the name of the cation, cobalt. Therefore, the name of this compound is cobalt (III) oxide.
(c) In this compound of cobalt, we see that we need two atoms of oxygen (O) for every atom of cobalt. If oxygen has an oxidation number of $2-$, then we know this atom of cobalt has an oxidation number of $4+$ (the ratio is reduced from four atoms of oxygen for every two atoms of cobalt to two atoms of oxygen for every atom of cobalt). We represent this oxidation number with the Roman numeral IV in parentheses following the name of the cation, cobalt. Therefore, the name of this compound is cobalt (IV) oxide.
