The oxidation number of calcium is two. The oxidation number of calcium refers to the number of electrons lost by an atom when a chemical compound is being created. A molecule's oxidation number, also called an oxidation state, is a hypothetical change based on the idea of an atom's bonds all being ionic, which rarely happens.
The oxidation state is a way to determine the number of electrons being added or removed from an atom. When electrons are removed, creating a positive atom, then the atom has been oxidized. If electrons are added, making a negative atom, then the atom has gone through reduction. There are several rules which are used as a standard to determine the oxidation state of an atom. The oxidation state of a pure element is always going to be zero and the structure of the element has no effect on the oxidation state of the given element. The oxidation state in a neutral compound is always going to be zero. In compounds that are not neutral the oxidation state is always equal to the charge on the ion or atom. There are compounds and elements that have exceptions to the normal oxidation state rules, including oxygen, hydrogen and chlorine, along with several others. These exceptions mean that the oxidation state may not follow the usual rules that guide the determination process.