The correct formula for the ionic compound formed between cobalt and oxygen is CoO, not Co2O. Here's why:

* Cobalt (Co) is a transition metal that can form multiple oxidation states. In this case, it forms a +2 charge (Co²⁺).

* Oxygen (O) is a nonmetal that forms a -2 charge (O²⁻).

To form a neutral compound, the charges must balance. Since the charges are already equal and opposite, the simplest ratio is 1:1.

Therefore, the correct formula is CoO, which is named Cobalt(II) oxide.