Which choice correctly identifies the oxidation number (o.n.) of sulfur in the compound Na2SO4?

• A. +4
• B. +6
• C. -2
• D. 0

Answer: (B)

Verified step by step guidance

1. Recall that the oxidation number (o.n.) of sodium (Na) in compounds is always +1 because it is an alkali metal.
2. Recognize that the sulfate ion (SO4) has an overall charge of -2, so the sum of the oxidation numbers of sulfur and oxygen in SO4 must equal -2.
3. Know that oxygen almost always has an oxidation number of -2 in compounds (except in peroxides or when bonded to fluorine). Since there are 4 oxygen atoms, their total contribution to the charge is 4 × (-2) = -8.
4. Set up the equation for the sum of oxidation numbers in the sulfate ion: let the oxidation number of sulfur be x, then x + (-8) = -2.
5. Solve the equation for x: x = -2 + 8, which gives the oxidation number of sulfur in Na2SO4.