Explain why the symbol for an atom of the element oxygen and the formula for a molecule of oxygen differ.

See below.

Explanation:

In the scientific setting, a few elements exist in monoatomic state, meaning their atoms could persist as single isolated atoms. The symbol for oxygen's atom is O which actually is the symbol for monoatomic oxygen.

In order to gain a stable electronic configuration, two oxygen atoms share electrons with each other to form covalent bonds. Hence, we get a entity which consists of 2 oxygen atoms with a covalent bond which is O2, molecule of oxygen.