In redox reactions, why is the term 'reduction' used to describe the gain of electrons by a species?

• A. Because the species loses energy during electron gain.
• B. Because the mass of the species decreases when it gains electrons.
• C. Because the number of protons in the species is reduced.
• D. Because the oxidation state of the species is reduced when it gains electrons.

Answer: (D)

Verified step by step guidance

1. Understand that in redox reactions, the terms 'oxidation' and 'reduction' refer to changes in the oxidation state of a species.
2. Recall that 'reduction' specifically means the gain of electrons by a species, which causes its oxidation state to decrease (become more negative or less positive).
3. Recognize that the term 'reduction' comes from this decrease or 'reduction' in the oxidation number, not from changes in mass, energy, or proton count.
4. Note that gaining electrons adds negative charge to the species, lowering its oxidation state, which is why the process is called 'reduction'.
5. Therefore, the correct explanation is that the species is said to be reduced because its oxidation state is reduced when it gains electrons.