What Is a Molecule? Definition and Examples

What Is a Molecule? Definition and Examples
A molecule is an electrically neutral group of atoms held together by chemical bonds.

A molecule is defined as an electrically neutral group of two or more atoms connected by chemical bonds. Here are examples of molecules and a look at the difference between a molecules, compounds, and ions.

Atom vs Molecule

An atom is the smallest unit of an element. All atoms of a single element contain the same number of protons. Atoms cannot be further divided using any chemical method. In contrast, a molecule is the unit of a pure substance. A molecule consists of more than one atom. A given molecule has a constant composition, which means the ratio of different atoms is always the same. Identical molecules also display the same chemical properties. Unlike atoms, molecules may be broken by chemical reactions. Breaking all of the chemical bonds in a molecule results in a collection of atoms.

Examples of Molecules

A molecule may consist of two or more atoms of a single element or atoms of different elements. Here are some examples of molecules:

  • H2O (water)
  • N2 (nitrogen)
  • O3 (ozone)
  • CaO (calcium oxide)
  • CO2 (carbon dioxide)
  • C6H12O6 (glucose, a type of sugar)
  • NaCl (table salt)
  • Hemoglobin (C738H1166N812O203_S2Fe)
  • Vitamin C or ascorbic acid (C6H8O6)
  • Deoxyribonucleic acid (DNA)

Types of Molecules

Molecules are classified according to their composition:

  • Diatomic molecule – A diatomic molecule consists of only two atoms. These may be atoms of a single element (homonuclear) or atoms of different elements (heteronuclear). O2 is an example of a homonuclear diatomic molcule. HCl is an example of a heteronuclear diatomic molecule.
  • Polyatomic molecule – A polyatomic molecule consists of more than two atoms. Glucose (C6H12O6) and water (H2O) are examples of polyatomic molecules.
  • Macromolecule – A macromolecule is a very large molecule, often consisting of subunits. Proteins and DNA are examples of macromolecules.

Another way to classify molecules is by their chemical bonds.

  • Covalent molecule – Covalent molecules consist of atoms connected by covalent chemical bonds. Covalent molecules consist of nometals. In a pure covalent molecule, the atoms have the same electronegativity values. Examples of pure covalent molecules are H2 and O3. In polar covalent molecules, the atoms have slightly different electronegativity values. Examples of polar covalent molecules are water (H2O) and hydrochloric acid (HCl).
  • Ionic molecule – Ionic molecules consist of both metals and nonmetals (with a few exceptions). The cation (first part of the molecule) and anion (second part of the molecule) have very different electronegativity values. Ionic molecules display extreme polarity, but usually when someone talks about a polar molecule, they mean a polar covalent molecule. Examples of ionic molecules include salt (NaCl), ammonium acetate (NH4CH3CO2 – an ionic compound consisting only of nonmetals), and sodium hydroxide (NaOH).

Difference Between a Molecule and a Compound

A compound consists of two or more different elements joined by chemical bonds. water (H2O) and hydrochloric acid (HCl) are compounds. Hydrogen gas (H2) and ozone (O3) are molecules, but not compounds. All compounds are molecules but not all molecules are compounds.

Difference Between a Molecule and an Ion

A molecule is electrically neutral. The total number of protons and electrons is the same. An ion contains a different number of protons and electrons. If there are more protons, the ion carries a positive charge. If there are more electrons, the ion carries a negative charge. An ion can start as an atom (O2-) or a molecule (H3O+). You can tell it apart from a molecule because it always has a + or – superscript for its charge.

What Is Not a Molecule?

Examples of substances that are not molecules are atoms and ions:

  • H (hydrogen atom)
  • Ag (silver atom)
  • PO4 (phosphate ion)

Additionally, alloys aren’t exactly molecules or compounds, either. This is because an alloy consists of a mixture of metals and nonmetals. The elements may form some chemical bonds, but they don’t associate in a fixed mole ratio.


  • Brown, T.L.; Kenneth C. Kemp; Theodore L. Brown; Harold Eugene LeMay; Bruce Edward Bursten (2003). Chemistry – The Central Science (9th ed.). New Jersey: Prentice Hall. ISBN 978-0-13-066997-1.
  • Chang, Raymond (1998). Chemistry (6th ed.). New York: McGraw Hill. ISBN 978-0-07-115221-1.
  •  IUPAC (1997). “Molecule.” Compendium of Chemical Terminology (2nd ed.) (the “Gold Book”). Oxford: Blackwell Scientific Publications. ISBN 0-9678550-9-8. doi:10.1351/goldbook
  • Zumdahl, Steven S. (1997). Chemistry (4th ed.). Boston: Houghton Mifflin. ISBN 978-0-669-41794-4.

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