Final answer:
Oxidation numbers are assigned using a set of rules: O2 has an oxidation number of 0 for each O atom; in H2O, H is +1 and O is -2; NaCl has Na at +1 and Cl at -1; and in SO3, each O is -2, with S at +6.
Step-by-step explanation:
Assigning oxidation numbers to elements in compounds and ions follows a set of rules to simplify the process. Here's how to assign oxidation numbers for each given substance:
- For O2, since it is in its elemental form, each oxygen atom has an oxidation number of 0 by rule 1.
- In H2O, the oxygen atom is assigned an oxidation number of -2 by rule 3, and hydrogen, with its +1 oxidation number, makes the sum of the oxidation numbers equal to the neutral charge of the molecule.
- For NaCl, sodium (Na) as an alkali metal has an oxidation number of +1 according to rule 2, and chlorine (Cl) has an oxidation number of -1, balancing to zero, which is the charge of the compound.
- In SO3, each oxygen has an oxidation number of -2. Therefore, the sulfur (S) must have an oxidation number of +6 to make the sum of the oxidation numbers equal to zero (rule 4).
This is how we apply the rules of assigning oxidation numbers to determine the states of different elements in various substances.