Oxidation numbers provides a means of keeping track of electrons in redox reactions. For some elements, the oxidation number is just another way of stating what its most stable cation or anion will be. In other cases, it is not so obvious, so we have rules.
Rules for assigning oxidation numbers
- The oxidation number for elements is always zero. For example, Na(s), O2(g), C(s) all have zero oxidation numbers.
- The oxidation number of monoatomic ions is the same as their charge. You already know this one. This means that for Na+, the oxidation number is +1 and for Cl-, the oxidation number is -1.
- Oxygen is assigned a -2 oxidation number in covalent compounds. This refers to compounds such as CO, CO2, SO2, and SO3. There is an exception to this rule, and it is in peroxides, such as H2O2. Here, each O in the O22- group has a -1 oxidation number.
- Hydrogen is assigned a +1 oxidation number in covalent compounds. This refers to compounds such as HCl, NH3, and H2O.
- In binary compounds, the element with the greatest attraction to electrons gets the negative oxidation number. In other words, the most electronegative of the pair gets the negative number. For example, in HF, F is more electronegative and thus has a -1 oxidation number. In NH3, the N atom is more electronegative and has a -3 oxidation number.
- The sum of the oxidation numbers is zero for a neutral compound and equal to the ion's charge for an ionic species. For example, in H2O, a neutral species, H is +1 and O is -2, and the sum of the two is 0. For CO32-, each O is -2 and C is +4, and the sum is -2.
Assign oxidation numbers to the atoms in SF6.
Since this is a binary compound let's first start with rule 5. We know that F has a greater attraction to electrons than S does, therefore we give it the negative oxidation number, which in this case will be -1 for F. To assign sulfur its oxidation number, we go to rule 6. This is a neutral compound, so the sum of the total oxidation numbers must be zero. Since there are 6 F atoms, each with a -1 oxidation state then the sulfur must have a +6 oxidation number to balance out the fluorine atoms.
Chemisty, The Central Science, 10th Ed.
2.49, 2.50, 2.51, 2.52