Final answer:
Oxidation numbers of atoms can be determined by considering the total number of valence electrons, arranging the atoms to follow the octet rule, and distributing the electrons accordingly.
Step-by-step explanation:
Oxidation numbers can be determined by considering the total number of valence electrons in a molecule or ion and arranging the atoms to follow the octet rule. The number of valence electrons can be found by referring to the periodic table. For example, oxygen has 6 valence electrons and hydrogen has 1 valence electron. By distributing the electrons and following the octet rule, the oxidation numbers can be determined.
For example:
In the molecule CO3^2-, the total number of valence electrons is 24. Oxygen has an oxidation number of -2, so each oxygen atom will contribute -6. Since the overall charge of the ion is -2, the carbon atom must have an oxidation number of +4 in order to balance out the negative charge from the oxygens.