The term "anion" refers to the fact that in an electric field, such as during electrolysis, it always moves towards the positively charged anode. Anions are formed from atoms or molecules by accepting electrons or giving off hydrogen ions H+ (protons). Anions, e.g. oxide (O2-) or sulphide (S2-) anions, can form a variety of compounds by combining with different cations. Therefore, whole classes of compounds are derived from the individual anions, in this example the oxides and sulfides, which form many ores and rocks and are therefore particularly important in mineralogy and metallurgy. In pharmaceutical terminology, the terms "anionic emulsions" and "anionic surfactants" are used for mixtures of substances that have a negatively charged functional group.