Final answer:
When an atom undergoes reduction, its oxidation state decreases due to the gain of electrons. This decrease in oxidation number signifies that during a redox reaction, the species that is reduced gains electrons, as indicated by the mnemonic 'oil rig'.
Step-by-step explanation:
When an atom undergoes reduction, its oxidation state decreases. This means that the atom gains electrons, which are negatively charged, resulting in a lower oxidation number. A redox reaction, which stands for reduction-oxidation reaction, always involves both processes happening simultaneously; while one species is reduced, another one is oxidized. During the reduction half of the reaction, the oxidation number of the atom is reduced, demonstrating that it has undergone a gain of electrons. This concept follows the mnemonic 'oil rig', which means 'oxidation is loss, reduction is gain' of electrons.
For example, when copper(I) oxide (Cu₂O) reacts with hydrogen gas (H₂), the copper is reduced, going from an oxidation state of +1 in the oxide to 0 in the elemental copper produced, while the hydrogen is oxidized from 0 to +1 as it forms water.