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.
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
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