Oxide mass percentage identifies the Period 3 element Steps:

• Mass of oxygen in 5.00g R: 5.00g - 2.50g T = 2.50g oxygen.
• Oxygen percentage: (2.50g / 5.00g) × 100% = 50%.
• Period 3 oxides' oxygen percentages: MgO (40%), Al₂O₃ (47%), SiO₂ (53%), SO₂ (50%).
• 50% matches SO₂, so T is sulfur. Why D is correct:
• SO₂ formula shows equal atomic masses (S=32, 2O=32), yielding exactly 50% oxygen by mass. Why the others are wrong:
• A: MgO oxygen percentage is 16/40 = 40%.
• B: Al₂O₃ oxygen percentage is 48/102 ≈ 47%.
• C: SiO₂ oxygen percentage is 32/60 ≈ 53%. Final answer: D