All atoms within any given substance have an oxidation state. The oxidation state is a number (usually an integer) that can be positive, negative, or zero. The oxidation state of an atom provides a measure of the degree of oxidation of that atom as it exists in that substance. Such information is useful to the chemist in determining how the substance will react or has reacted.
 

Study the graphic below in order to learn about oxidation states and the rules for assigning oxidation states. You will find a simple rule in the listing for a monatomic ions that apply to the two ions of this binary ionic compound.