Oxidation Number Changes in Redox Reaction

Steps:

• Assign oxidation number to sulfur: 0 in elemental S, +4 in SO₂ (2O at -2 each, so S +4 for neutral molecule).
• Calculate sulfur change: +4 - 0 = +4 (oxidation).
• Assign oxidation number to nitrogen: +5 in HNO₃ (H +1, 3O at -6, neutral so N +5), +4 in NO₂ (2O at -4, neutral so N +4).
• Calculate nitrogen change: +4 - (+5) = -1 (reduction).

Why D is correct:

• Matches definition of oxidation number change: sulfur loses 4 electrons (+4 shift), nitrogen gains 1 electron per atom (-1 shift) in balanced redox.

Why the others are wrong:

• A: Sulfur change +4, not +2; nitrogen -1, not -3.
• B: Nitrogen change -1, not -3.
• C: Nitrogen change -1, not -3.

Final answer: D