A molecule is defined as an electrically neutral group of two or more atoms held together by covalent chemical bonds that acts as a single unit. Molecules range from simple substances such as oxygen (O₂) and water (H₂O) to enormous biological molecules like DNA and proteins. Understanding what a molecule is helps explain chemical reactions, the properties of matter, and how atoms combine to form everything from air and water to living cells.
Key Takeaways: What Is a Molecule?
- A molecule is a neutral group of two or more atoms joined by covalent chemical bonds.
- Molecules may contain atoms of the same element or different elements.
- Every compound made of discrete covalent units is a molecule, but not every molecule is a compound.
- Ionic compounds such as sodium chloride (NaCl) are generally not molecules because they form crystal lattices rather than individual molecular units.
- Molecules may be tiny, like hydrogen (H₂), or extremely large, like proteins and DNA.
- Chemical reactions create, rearrange, or break molecules by making and breaking chemical bonds.
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:
| Molecule | Description |
|---|---|
| H₂ | Hydrogen gas |
| O₂ | Oxygen gas |
| O₃ | Ozone |
| H₂O | Water |
| CO₂ | Carbon dioxide |
| NH₃ | Ammonia |
| CH₄ | Methane |
| HCl | Hydrogen chloride |
| C₆H₁₂O₆ | Glucose |
| C₆H₆ | Benzene |
| DNA | Biological macromolecule |
| Hemoglobin | Protein containing thousands of atoms |
Not molecules: NaCl, MgO, CaO, SiO₂ (quartz), and most metals because they form extended crystal structures instead of discrete molecules.
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 molecule. 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. Most molecules are held together by covalent bonds, although the polarity of those bonds varies.
Molecules by Bond Type
Most molecules are held together by covalent bonds, although the polarity of those bonds varies.
Nonpolar covalent molecules
Atoms share electrons nearly equally.
Examples:
- H₂
- N₂
- O₂
- CH₄
Polar covalent molecules
Atoms share electrons unequally, creating partial positive and negative charges.
Examples:
- H₂O
- NH₃
- HCl
- SO₂
Covalent molecules consist of atoms connected by covalent chemical bonds. Covalent molecules consist of nonmetals. 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).
Although ionic compounds contain chemical bonds, substances such as sodium chloride (NaCl) do not exist as individual molecules. Instead, they form giant three-dimensional crystal lattices composed of ions.
Difference Between a Molecule and a Compound
The terms molecule and compound are closely related but not identical.
| Molecule | Compound |
|---|---|
| Two or more atoms bonded together | Substance containing two or more different elements |
| May contain one element | Always contains at least two elements |
| May or may not be a compound | May be molecular or ionic |
| Examples: O₂, O₃, H₂O | Examples: H₂O, CO₂, NaCl |
Water is both a molecule and a compound because it consists of different elements joined by covalent bonds.
Oxygen gas (O₂) is a molecule but not a compound because it contains only one element.
Sodium chloride is a compound but not a molecule because it forms an ionic crystal lattice.
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.
Polyatomic ions contain covalent bonds internally but carry an overall electrical charge, so they are not molecules.
Atom vs Molecule vs Compound vs Ion
| Feature | Atom | Molecule | Compound | Ion |
|---|---|---|---|---|
| Smallest unit? | Yes | No | No | No |
| Neutral? | Usually | Yes | Usually | No |
| Contains chemical bonds? | No | Yes | Yes | Sometimes |
| May contain one element? | Yes | Yes | No | Yes |
| Has electrical charge? | No | No | No | Yes |
| Example | He | O₂ | NaCl | Na⁺ |
What Is Not a Molecule?
Not all substances are made of molecules.
Examples include:
- Individual atoms (He, Ne, Ag)
- Positive and negative ions (Na⁺, Cl⁻, PO₄³⁻)
- Ionic crystals (NaCl, MgO, CaO)
- Metals (Cu, Fe, Al)
- Covalent network solids such as diamond, graphite, and quartz (SiO₂)
These materials consist of continuous atomic or ionic networks rather than separate molecular units.
How Molecules Form
Molecules form when atoms share one or more pairs of electrons through covalent bonds. Sharing electrons allows the atoms to achieve more stable electron configurations, often satisfying the octet rule (or duet rule for hydrogen).
For example:
- Two hydrogen atoms share one pair of electrons to form H₂.
- One oxygen atom shares electrons with two hydrogen atoms to form H₂O.
- Carbon forms four covalent bonds, allowing it to build millions of different molecules.
Chemical reactions create new molecules by breaking old covalent bonds and forming new ones.
Common Misconceptions
Salt is made of NaCl molecules.
No. Table salt consists of an enormous lattice of sodium and chloride ions rather than individual NaCl molecules.
Every compound is a molecule.
No. Ionic compounds such as NaCl are compounds but not molecules.
Every molecule contains different elements.
No. Oxygen (O₂), nitrogen (N₂), ozone (O₃), and sulfur (S₈) are molecules composed of only one element.
Large molecules are rare.
No. Biological molecules such as DNA, proteins, and cellulose contain thousands or even millions of atoms.
Frequently Asked Questions
What is the simplest molecule?
Hydrogen gas (H₂) is generally considered the simplest stable molecule.
Can one atom be a molecule?
No. A molecule must contain at least two atoms.
Are all gases molecules?
No. Noble gases such as helium and neon exist as individual atoms rather than molecules.
Is water a molecule?
Yes. Water is both a molecule and a compound.
Is table salt a molecule?
No. Table salt is an ionic compound composed of a crystal lattice of Na⁺ and Cl⁻ ions.
What is the largest known molecule?
Scientists have synthesized molecules containing thousands of atoms, while naturally occurring biomolecules such as DNA can contain millions of atoms
References
- 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.