How is a molecule defined?

A molecule is defined as two or more atoms held together by covalent bonds. This definition highlights the nature of bond formation involved in molecules, specifically the sharing of electron pairs between atoms. Covalent bonding typically occurs between nonmetal atoms, forming stable structures as they reach a more energetically favorable state together than they would as individual atoms.

In contrast, a collection of ions refers to charged particles that are not bound by covalent bonds, but rather by ionic interactions, which is not consistent with the definition of a molecule. Additionally, bonding through ionic means involves transferring electrons from one atom to another, leading to the formation of charged ions instead of a stable molecule.

While two or more atoms held together by ionic bonds align more with what is seen in ionic compounds rather than molecules, a single atom of an element fails to meet the criteria of multiple atoms being involved. The essence of a molecule is that it consists of multiple atoms bonded together in a stable form, specifically through covalent bonds. Thus, the correct choice encapsulates the fundamental characteristics of molecular formation and structure.