Periodic table with oxidation numbers

Oxidation states and ion charges

Ion charges describe real ions, such as Na⁺ in table salt. Oxidation states are a bookkeeping tool that works for every compound, including covalent ones in which no ions exist. In carbon dioxide, CO₂, carbon is given +4 and each oxygen −2, even though there is no C⁴⁺ ion. Oxidation states let you follow electrons in redox reactions: an atom whose oxidation state goes up is oxidized, and one whose oxidation state goes down is reduced.

Rules for assigning oxidation states

An element on its own, such as O₂ or Fe, is 0. A simple ion has an oxidation state equal to its charge. Fluorine is always −1 in compounds. Oxygen is usually −2, but −1 in peroxides such as H₂O₂. Hydrogen is usually +1, but −1 in metal hydrides such as NaH. Group 1 metals are +1 and group 2 metals +2. The oxidation states in a neutral compound add up to 0, and in a polyatomic ion they add up to the ion's charge.

The table shows only the most common oxidation states. Oxygen is listed as −2 only, and the noble gases as 0, even though xenon reaches +2, +4 and +6 in its fluorides. For the superheavy elements from meitnerium onward no values are known, so their cells show a dash.

Patterns across the table

For main-group elements, the highest positive oxidation state usually equals the number of valence electrons, and the most negative equals that number minus 8. Nitrogen, with 5 valence electrons, ranges from +5 to −3, and chlorine, with 7, is listed from +7 down to −1. Metals on the left show only positive values: +1 for group 1 and +2 for group 2.

Transition metals can use d electrons as well as their outer s electrons, so many have several oxidation states. Manganese is listed with +2, +3, +4 and +7, and chromium with +2, +3 and +6. Their ability to switch between oxidation states is one reason they make good catalysts.

Worked example: manganese in KMnO₄

Potassium permanganate, KMnO₄, is neutral, so its oxidation states must add up to 0. Potassium is a group 1 metal, so it is +1, and the four oxygen atoms are −2 each, giving −8. That leaves +1 + Mn − 8 = 0, so manganese is +7, the highest value listed for it. The same method works for sulfuric acid, H₂SO₄: 2 × (+1) + S + 4 × (−2) = 0, so sulfur is +6.