Rule for assing oxidation number with examples
1. The oxidation number of an atom in its elemental form is always 0.
Example: In O2, the oxidation number of each oxygen atom is 0.
2. The oxidation number of a monatomic ion is equal to the charge of the ion.
Example: In the ion Na+, the oxidation number of sodium is +1.
3. In a neutral molecule, the sum of the oxidation numbers of all atoms is 0.
Example: In H2O, the oxidation number of hydrogen is +1 and the oxidation number of oxygen is -2, which adds up to 0.
4. In polyatomic ions, the sum of the oxidation numbers of all atoms is equal to the charge of the ion.
Example: In the sulfate ion (SO4^2-), the oxidation number of sulfur is +6, the oxidation number of each oxygen atom is -2, and the sum of the oxidation numbers equals -2.
5. Group 1 elements have an oxidation number of +1 in compounds.
Example: In NaCl, the oxidation number of sodium is +1.
6. Group 2 elements have an oxidation number of +2 in compounds.
Example: In MgCl2, the oxidation number of magnesium is +2.
7. Fluorine always has an oxidation number of -1 in compounds.
Example: In HF, the oxidation number of fluorine is -1.
8. In compounds, oxygen usually has an oxidation number of -2.
Example: In H2O, the oxidation number of oxygen is -2.
9. Hydrogen usually has an oxidation number of +1 in compounds, except in metal hydrides where it has an oxidation number of -1.
Example: In CH4, the oxidation number of hydrogen is +1.