Molecule

The smallest particle of a substance that has all of the physical and chemical properties of that substance. Molecules are made up of one or more atoms. If they contain more than one atom, the atoms can be the same (an oxygen molecule has two oxygen atoms) or different (a water molecule has two hydrogen atoms and one oxygen atom). Biological molecules, such as proteins and DNA, can be made up of many thousands of atoms.

The stable union of two or more atoms; some organic molecules contain very large numbers of atoms.

The smallest particle of a substance that retains the properties of the substance and is composed of one or more atoms.

Chemical combination of two or more atoms of the same chemical element (such as O₂) or different chemical elements (such as H₂O).