Question

Which rule for assigning oxidation numbers is correct?
Hydrogen is usually –1.
Oxygen is usually –2.
A pure group 1 element is +1.
A monatomic ion is 0.

Answer 1

Only the statement "Oxygen is usually −2" is correct as written.

Corrections and explanations:
- "Hydrogen is usually −1." — Incorrect. Hydrogen is usually +1 when bonded to nonmetals (H2O: H = +1) and −1 when bonded to metals (metal hydrides, e.g. NaH: H = −1).
- "A pure group 1 element is +1." — Incorrect. A pure (elemental) substance has oxidation number 0. Group 1 elements have oxidation number +1 when they are in compounds (e.g. Na in NaCl is +1).
- "A monatomic ion is 0." — Incorrect. A monatomic ion has an oxidation number equal to its ionic charge (Cl− is −1, Fe3+ is +3). Only a neutral atom (elemental monatomic species) has oxidation number 0.

Useful general rules: the sum of oxidation numbers in a neutral compound is 0; in a polyatomic ion the sum equals the ion charge.