Magnesium is the pure element, however it is not found in nature by itself, it always bonds with another element. So you have many forms of magnesium such as magnesium Aspartate, bicarbonate,...
Magnesium ion, usually in Mg+2 oxidation state is a magnesium atom with 2 absent electrons in the valence shell. Magnesium atom is in a neutral (0) oxidation state, which still contains the 2...