What rule determines the oxidation state for elemental atoms?

Elemental atoms have an oxidation state of zero because they are in their natural, uncombined form. This means that they exist as pure elements, such as O₂ (oxygen gas) or Na (sodium metal), rather than bonded to other elements. In this state, there is no net loss or gain of electrons; the number of electrons equals the number of protons, leading to an overall charge of zero.

When considering oxidation states, it is important to recognize that the rule applies universally to all elemental forms, including diatomic gases and noble gases. Thus, any pure element will always have an oxidation state of zero when it is not part of a compound. This foundational concept is critical for understanding more complex oxidation states encountered in compounds and reactions.