Atoms vs Molecules vs Compounds vs Elements

“Is water an element or a compound?” “Is oxygen gas a molecule or an element?” “Is salt a molecule?” These four words, atom, element, molecule and compound, trip up almost every chemistry student at some point, partly because they overlap. Oxygen gas, for example, is both an element and made of molecules. This guide untangles them with clear definitions, a comparison table and plenty of examples.

The four definitions

Atom

An atom is the smallest particle of an element that still has that element’s chemical identity. It has a tiny nucleus of protons and neutrons surrounded by electrons. (For the full picture, see what is an atom?.)

Examples: a single carbon atom, a single helium atom, a single gold atom.

Element

An element is a pure substance made of only one kind of atom, meaning all its atoms have the same number of protons (the same atomic number). There are 118 known elements, and every one of them is on the periodic table.

Examples: a lump of iron (all Fe atoms), helium gas (all He atoms), oxygen gas (all O atoms).

Molecule

A molecule is a group of two or more atoms held together by covalent bonds, acting as a single unit. The atoms can be the same element or different elements.

Examples: O₂ (two oxygen atoms), H₂O (two hydrogens and one oxygen), CO₂, C₆H₁₂O₆ (glucose).

Compound

A compound is a pure substance made of two or more different elements chemically combined in a fixed ratio. A compound has its own properties, usually very different from the elements that form it.

Examples: water (H₂O), carbon dioxide (CO₂), sodium chloride (NaCl), methane (CH₄).

The key idea: two different questions

Most confusion disappears once you realise these words answer two different questions:

1. What is it made of? One kind of atom → element. Two or more kinds chemically bonded → compound.
2. What are its particles like? Separate atoms, discrete molecules, or a giant network (lattice)?

“Molecule” belongs to question 2. “Element” and “compound” belong to question 1. So a substance can be described by one word from each list.

Comparison table

	Atom	Element	Molecule	Compound
What it is	A single particle	A pure substance	A bonded group of atoms	A pure substance
Kinds of atoms	One	One	One or more	Two or more
Chemically bonded?	Not to others (by definition, a single atom)	Maybe (O₂ yes, He no)	Yes, covalently	Yes
Fixed composition?	Yes	Yes	Yes	Yes, in a fixed ratio
Can it be separated physically?	No	No	No (only by breaking bonds)	No (only by chemical reaction)
Examples	One Na atom	Sodium metal, O₂ gas	O₂, H₂O, CO₂	H₂O, NaCl, CO₂

Combining the words: the four possibilities

	Made of molecules	Not made of molecules
Element	O₂, N₂, H₂, Cl₂, P₄, S₈	He, Ne, Ar (single atoms); Fe, Cu, C as diamond (giant structures)
Compound	H₂O, CO₂, NH₃, C₆H₁₂O₆	NaCl, MgO, SiO₂ (giant lattices)

This table answers most of the usual trick questions:

• Oxygen gas (O₂) is an element (only oxygen atoms) made of molecules. It is not a compound, because only one element is present.
• Water (H₂O) is a compound made of molecules.
• Helium is an element made of single atoms, not molecules.
• Sodium chloride (NaCl) is a compound, but it is not made of molecules. It is a giant lattice of Na⁺ and Cl⁻ ions. “NaCl” is a formula unit showing the 1:1 ratio, not a molecule.

The diatomic elements article lists the seven elements that exist naturally as two-atom molecules.

Where mixtures fit in

A mixture contains two or more substances that are not chemically bonded to each other. Each keeps its own properties, and the proportions can vary.

	Compound	Mixture
Chemically bonded?	Yes	No
Fixed proportions?	Yes	No
Properties	New, different from the elements	Similar to the components
Separation	Only by chemical reactions	By physical methods (filtering, distilling, chromatography)
Example	Water, H₂O	Air, salt water, brass

Air is a good example of a mixture that contains both elements and compounds: nitrogen (N₂) and oxygen (O₂) are elements made of molecules, while carbon dioxide (CO₂) and water vapour are compounds. None of them are bonded to each other.

A quick decision flowchart

Ask these questions in order:

1. Is there more than one substance, not bonded together? → Mixture. (Stop.)
2. Is it one pure substance with only one kind of atom? → Element.
3. Is it one pure substance with two or more kinds of atoms bonded in a fixed ratio? → Compound.
4. Then, separately: are its particles discrete bonded groups? → it’s made of molecules. If it’s single atoms or a giant lattice, it isn’t.

An everyday analogy

Think of LEGO:

• An atom is a single brick.
• An element is a box containing only one colour of brick, all red, for example.
• A molecule is a small model built from bricks snapped together, such as a two-brick red stack or a car made of red, blue and white bricks.
• A compound is a box full of identical models that each contain more than one colour.
• A mixture is a box with a jumble of different models and loose bricks, in no particular ratio.

The analogy isn’t perfect (real bonds involve electrons, not studs), but it captures the relationships.

Tricky cases and common mistakes

• “All molecules are compounds.” No. O₂, N₂ and O₃ are molecules of elements.
• “All compounds are made of molecules.” No. Ionic compounds like NaCl and giant covalent compounds like SiO₂ (quartz) form lattices, not molecules. You can read more in ionic vs covalent bonds.
• “Diamond is a compound because it has lots of atoms bonded together.” Diamond contains only carbon, so it’s an element, even though its atoms are covalently bonded in a giant network. Diamond and graphite are two forms of the same element (see allotropes of carbon).
• “A compound has the properties of its elements.” Sodium is a reactive metal and chlorine a toxic gas, yet sodium chloride is table salt. New properties are a hallmark of compounds.
• “An ion is a kind of atom, so it’s an element.” An ion is a charged atom or group of atoms. Na⁺ is a sodium ion; it’s still the element sodium, but it only exists alongside a negative partner in a compound (see what is an ion?).

Practice questions

Classify each as element or compound, and say whether it is made of molecules.

1. Nitrogen gas, N₂
2. Carbon dioxide, CO₂
3. Copper wire
4. Magnesium oxide, MgO
5. Ozone, O₃
6. Glucose, C₆H₁₂O₆
7. Neon gas

Answers

1. Element, made of molecules.
2. Compound, made of molecules.
3. Element, not molecules (a metallic lattice).
4. Compound, not molecules (an ionic lattice).
5. Element, made of molecules (an allotrope of oxygen).
6. Compound, made of molecules.
7. Element, single atoms, not molecules.

Key takeaways

• An atom is the smallest unit of an element.
• An element contains only one kind of atom; a compound contains two or more elements chemically bonded in a fixed ratio.
• A molecule is a covalently bonded group of atoms. It can be an element (O₂) or a compound (H₂O).
• Ionic compounds and giant covalent substances are not made of molecules; they form lattices.
• A mixture contains substances that aren’t chemically bonded and can be separated physically.
• Browse every element, with its atomic structure and common compounds, on the elements page.