What is the oxidation number of an atom in its elemental state?

In chemistry, the oxidation number (or oxidation state) of an element in its elemental state is always considered to be zero. This applies to single atoms that are not combined with other different elements. For instance, in molecular oxygen (O2), nitrogen (N2), or elemental gold (Au), each atom does not have a charge; thus, their oxidation states are all zero.

This concept is fundamental because it serves as a baseline for understanding how oxidation states change when atoms combine to form compounds. The oxidation state is a theoretical construct that helps chemists keep track of electron transfer in redox reactions, making it essential to recognize that elemental forms do not exhibit any gain or loss of electrons, hence an oxidation state of zero.

Thus, in your question regarding the oxidation number of an atom in its elemental state, the correct answer is indeed zero.