What defines a molecule?

A molecule is specifically defined as two or more atoms that are held together by chemical bonds. This definition encompasses both covalent and some other forms of bonding where atoms interact to form a stable unit. For example, in water (H₂O), two hydrogen atoms bond with one oxygen atom to create a molecule through the sharing of electrons, which is a covalent bond. The formation of molecules is fundamental to the structure and function of all living organisms, as they are the building blocks of more complex structures, such as proteins, nucleic acids, and carbohydrates.

While a single atom of an element refers to an atom, it does not meet the criteria for a molecule, as it does not involve any bonding with other atoms. Similarly, a collection of ions pertains to ionic interactions and does not necessarily result in the formation of molecular bonds. Additionally, atoms lacking bonds between them do not form a stable entity and therefore cannot be classified as a molecule. Thus, the key characteristic that distinguishes a molecule is the presence of chemical bonds between atoms, which option B accurately describes.