Roman-numeral notation for oxidation numbers

An oxidation number is a number assigned to an element in a compound to represent its oxidation state. In a compound’s name, a Roman numeral in parentheses immediately after an element’s name gives its oxidation number.

For example, in iron(III) oxide, the numeral III tells us that iron has an oxidation number of $$+3$$. In iron(II) oxide, the numeral II tells us that iron has an oxidation number of $$+2$$.

The numeral gives the oxidation number, not the number of atoms. Iron(III) oxide has the formula $$\ce{Fe2O3}$$: each formula unit contains two iron atoms, not three. The III in its name identifies iron’s oxidation state.

Redox reactions by oxygen transfer

In some reactions, oxygen is transferred from one substance to another. The substance that gains oxygen is oxidised, while the substance that loses oxygen is reduced.

Oxidation and reduction happen at the same time in a redox reaction. Oxygen gained by one substance is lost by another.

Identifying oxidation and reduction

Compare the substances before and after the reaction. Follow what happens to each substance and check whether it has gained or lost oxygen.

For example, carbon reacts with copper oxide to produce carbon dioxide and copper:

Carbon starts without oxygen and ends as carbon dioxide. It has gained oxygen, so carbon is oxidised.

Copper oxide starts with oxygen and ends as copper. It has lost oxygen, so copper oxide is reduced.

Because oxidation and reduction occur together, this is a redox reaction.