What is a molecule?

A molecule is defined as two or more atoms that are held together by covalent bonds. This means that the atoms share electrons, resulting in a stable structure. Molecules can consist of the same type of atoms, such as oxygen (O2), or different types of atoms, such as water (H2O), where hydrogen and oxygen atoms are bonded together. The presence of covalent bonds is what distinguishes molecules from other types of chemical entities, such as ionic compounds or single atoms.

The other options are focused on different concepts: ionic bonds describe the interactions between charged ions rather than covalent bonds; a single atom alone does not form a molecule; and while a molecule can be a type of chemical compound if it consists of different elements, not all molecules (like diatomic oxygen) fit that classification. Thus, the focus of the correct answer illustrates the specific definition of a molecule as a collection of covalently bonded atoms.