1) The smallest particle of an element or compound capable of a stable, independent existence.
2) a complete chemical unit. While a molecule is often thought of as consisting of more than one atom, this is not always true. For instance, helium has only one atom per molecule. The oxygen molecule (O2) contains two atoms, as do chlorine (Cl2), hydrogen (H2) and nitrogen (N2). Some molecules are huge having a molecular weight in the millions. Macromolecules contain literally thousands of atoms. Some macromolecules must remain in tact in order to retain their chemical entity. These are usually proteins. Polymers are chains or networks of repeating sequences of chemical units known as molecules. Examples are polypropylene or cellulose. Molecules of polymers can be broken without destroying the chemical integrity.
3) A group of atoms linked together by covalent bonds.