Oxidation states and neutral formulas
Oxidation states are bookkeeping values for electrons. Formula subscripts are chosen so the total contribution in a neutral compound is zero.
Learning goals
- apply common oxidation-state rules;
- find an unknown value from a charge sum;
- construct and reduce neutral formulas;
- use parentheses correctly with polyatomic ions.
1. Meaning and rules
An oxidation state is an assigned number that helps track electron ownership. It is written with sign first, such as +2 or −1. A monatomic ion’s oxidation state equals its charge, but atoms in covalent compounds may have assigned oxidation states without being full ions.
| Rule | Example |
|---|---|
| Free element = 0 | Na, O₂, Cl₂: 0 |
| Monatomic ion = ion charge | Mg²⁺: +2; Cl⁻: −1 |
| Group 1 usually +1; group 2 usually +2 | NaCl; CaO |
| Fluorine in compounds = −1 | HF |
| Oxygen usually −2 | H₂O, CO₂ |
| Hydrogen usually +1 with nonmetals | HCl, H₂O |
| Sum = total species charge | neutral compound: 0; ion: ion charge |
2. Finding an unknown oxidation state
Let the unknown be $x$, multiply each oxidation state by its subscript, and set the sum equal to the species charge. In CO₂, oxygen contributes $2(-2)=-4$, so $x-4=0$ and carbon is +4. In $SO_4^{2-}$, $x+4(-2)=-2$, so sulfur is +6.
3. Constructing a formula from ions
- Write the cation first and anion second with charges.
- Find the smallest numbers of ions that make total charge zero. A least-common-multiple method is safest.
- Write those counts as subscripts; omit subscript 1.