Final answer:
Atoms exhibit an oxidation number of zero when they are part of a pure element because such atoms are not involved in chemical bonding that alters their original state. In contrast, atoms in an ionic compound have oxidation numbers equal to their ionic charges, balancing to a neutral overall charge.
Step-by-step explanation:
Atoms have an oxidation number of zero in a pure element. This is because a pure element consists of atoms of a single type without any charges or combinations that would modify their oxidation states. For example, the oxygen molecule O₂ is a diatomic molecule made up of oxygen atoms that are not ionically or covalently bonded to other elements, maintaining an oxidation state of zero.
An ionic compound, on the other hand, consists of positively charged cations and negatively charged anions in a ratio that results in no net electric charge. However, within the compound, each ion has an oxidation number equivalent to its charge. For example, in aluminum oxide (Al₂O₃), the aluminum cations are Al³⁺ and oxide anions are O²⁻, with respective oxidation numbers of +3 and -2.