What happens in a reduction reaction?

In a reduction reaction, a substance gains electrons, which decreases its oxidation state. This concept is central to redox (reduction-oxidation) reactions, where reduction is always accompanied by an oxidation, where another substance loses electrons. The transfer of electrons is fundamental in determining how elements combine and react with one another. Gain of electrons often leads to an increase in the negative charge of the species involved or a decrease in its oxidation state, making it more reduced.

The loss of protons does not directly relate to reduction, as protons are not the species undergoing transfer in this context. Similarly, gaining protons does not count as reduction since it does not involve electron transfer. Loss of electrons is actually the definition of oxidation, which contrasts with what happens during a reduction reaction. Therefore, the gain of electrons clearly defines the process of reduction in chemistry.