The __________ of an atom or molecule __________ when it undergoes reduction.

• A. oxidation number; increases
• B. atomic mass; decreases
• C. electron count; decreases
• D. oxidation number; decreases

Answer: (D)

Verified step by step guidance

1. Understand the concept of reduction in chemistry: Reduction is the gain of electrons by an atom or molecule.
2. Recall that the oxidation number (or oxidation state) represents the hypothetical charge an atom would have if all bonds were ionic.
3. When an atom or molecule gains electrons during reduction, its oxidation number decreases because electrons carry a negative charge.
4. Therefore, the oxidation number decreases upon reduction, not increases.
5. Confirm that other options do not fit: atomic mass does not decrease during reduction, and electron count actually increases, not decreases.