What is the formula of calcium oxide?

The chemical formula for calcium oxide is CaO. The logic behind it is pretty simple. Calcium ions carry a +2 charge, while oxygen ions have a -2 charge. Since their charges are equal and opposite, they balance each other out perfectly. This means you only need one of each atom to form the compound. That’s how you get the one-to-one ratio in CaO. It's also commonly known by the name quicklime.