Covalent bonds exist between two nonmetal elements while ionic bonds exist between a metal and a nonmetal element. In this case, oxygen is a nonmetal and in order to create an ionic bond, the other substance should be a metal. The answer therefore to this problem is calcium. read more
Many chemists today use an electronegativity difference of 2.0 as the lower cut off for a bond that behaves like an ionic bond. You can even compute the percent ionic character in a bond: % ionic character = 100 [1-e^-(DEN^2/4)] To test it out, use a difference in electronegativity (DEN) of 2.0. You should get a percent ionic character of 63.2 %. That means that the bond is 36.8 % covalent. read more