Calcium oxide is classified as an ionic compound. It is formed when calcium, which is a metal, loses electrons to oxygen, which is a nonmetal. The resulting compound is composed of positively charged calcium ions (Ca2+) and negatively charged oxide ions (O2-). These ions are held together by strong electrostatic forces, resulting in the ionic bond. The ionic nature of calcium oxide is further supported by its properties such as its high melting point, its brittleness, and its ability to conduct electricity when dissolved in water or melted.