The oxidation state of an atom is a description of how many electrons it has lost or gained from its original state. Each type of atom has a certain number of electrons (which varies from atom to atom) in its elemental form. When an atom forms a bond or otherwise interacts with another atom, it is possible that it will lose or gain an electron . If an atom is electronegative, it is more likely to take an electron away from another atom. If it is electropositive, it holds its own electrons weakly and is more likely to lose an electron when it interacts with another atomic species .
The oxidation number of an atom refers to the number of electrons it has lost or gained. Electrons are assigned a negative charge, so by convention if an atom has gained an electron, its oxidation number is reduced. For example, if an atom gains an electron, it has an oxidation number of -1. For this reason, an atom which receives an electron is described as having been "reduced." If an atom loses an electron, its oxidation number rises, and it is described as having been "oxidized." In an oxidation-reduction reaction , two atomic species interact so that one is reduced and one is oxidized.
An atom can be oxidized or reduced when a bond is formed. In a chemical bond , electrons are shared by two atoms . However, there is often one atom which is more electronegative than the other, and so holds electrons more tightly than the other. For this reason, one atom takes on a partial positive charge, and the other becomes partially negative. A bond such as this is called an ionic bond. In a very ionic bond, the electrons belong almost entirely to one atom. That atom has a new oxidation state of -1, and the atom which has "lost" its electron has an oxidation state of +1.