In a redox chemical reaction, one atom or group of atoms gains electrons while another atom or group of atoms loses them. The component that gains electrons is reduced while the component that loses them is oxidized. When an element suffers oxidation or reduction, the electron transferal sometimes results in the ionization of the element but sometimes a sharing of electrons takes place instead. In this latter type of redox reaction, the element does not become an ion but theoretically gains or loses electrons, depending on whether or not it attracts the shared electrons more strongly than its redox partner.
The oxidation number of an element represents the number of electrons that its atoms gain or lose in a redox reaction. Those elements that gain electrons have negative oxidation numbers. However, the oxidation number applies only after an element has reacted with another substance. In uncombined form, the oxidation state of an element is 0.
Many elements have different oxidation numbers, depending on the compound in which they occur. For example, hydrogen usually has an oxidation state of +1. However, it assumes a negative oxidation state of -1 when it forms a hydride, such as NaH, with a strongly electropositive metal. Oxygen usually has a negative oxidation number of -2, but it takes on an oxidation state of -1 in peroxides and +2 in the compound HF2.
The term "halogen" applies to the element chlorine because it enters into the composition of the mineral halide, which is table salt. Fluorine, bromine and iodine have chemical properties similar to chlorine, so they also are called halogens. These four elements usually gain one electron in redox reactions, so they normally have an oxidation number of -1. However, oxygen has a greater affinity for electrons than any halogen except fluorine so when united with oxygen in such compounds as HClO, chlorine has a positive oxidation number.
In general, nonmetals acquire negative oxidation numbers in most compounds, while the oxidation state of metals is regularly positive. The most electronegative nonmetals are oxygen, nitrogen and the four halogens mentioned above. The nonmetals carbon, sulfur, phosphorus and selenium have negative oxidation numbers in such compounds as methane, hydrogen sulfide, phosphine and sodium selenide, but their oxidation state is positive when they bond with such elements as oxygen.
In the periodic table, seven transitional elements lie between the metals and nonmetals. They usually resemble metals in their oxidation states. However, in some compounds, tellurium has an oxidation number of -2 and both arsenic and antimony sometimes assume a -3 oxidation state.