Short answer
An element is identified by one atomic number: all its atoms have the same number of protons. A compound is a substance in which two or more elements are chemically combined. Oxygen is an element; water is a compound containing hydrogen and oxygen. 1 2
Counting atoms is not enough. An oxygen molecule, O₂, contains two atoms but only one element. A water molecule, H₂O, contains three atoms representing two elements. The distinction is between kinds of elements, rather than between one atom and several. 3
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At a glance
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| Question | Element | Compound |
|---|---|---|
| Defining identity | One atomic number, determined by proton count. | Multiple elements chemically combined. |
| Can it contain many atoms? | Yes; an elemental sample ordinarily does. | Yes. |
| Can it consist of molecules? | Yes; O₂ is an example. | Yes; H₂O is an example, but not every compound consists of separate molecules. |
| Can ordinary chemical decomposition produce simpler substances? | It does not change the element into simpler elements. | A suitable chemical change can decompose a compound. |
| Does mixing substances necessarily make it? | No. | No; a mixture need not be a new compound. |
| Familiar example | Iron, Fe; oxygen, O₂. | Water, H₂O; sodium chloride, NaCl. |
Sources: elemental identity 1; substance classification 2; formulas and molecular examples 3; nonmolecular solids 4.
What each thing is
Element
An element is a type of atom defined by its proton count, also called its atomic number. Every carbon atom has six protons. Changing the number of neutrons gives a different isotope of carbon; changing the number of electrons gives an ion. Neither change by itself makes the atom a different element. 1
The word also names substances made from that element. A sample of elemental iron contains many iron atoms; “one element” does not mean one isolated atom. Oxygen can occur as bonded pairs, O₂, while still containing only oxygen atoms. 2 3
Compound
A compound combines different elements in a chemical substance with a characteristic composition and structure. Water and sodium chloride illustrate two familiar forms. Water consists of molecules; solid sodium chloride has an extended arrangement of positive and negative ions. Both are compounds, despite this structural difference. 2 3 4
Key differences
Chemical identity depends on how the elements are combined. Sodium chloride has properties different from free sodium and chlorine. Naming the ingredients does not imply that the finished compound retains the appearance or behavior of the uncombined elements. 2
A mixture is a separate possibility. Salt dissolved in water is a mixture containing substances derived from different elements, not a new compound called “saltwater.” The relative amount of salt can vary. Finding several elements in a sample does not, by itself, establish that the whole sample is one compound. 2
A formula needs to be interpreted correctly. O₂ has one element symbol and a subscript showing two atoms per molecule. H₂O has two element symbols. NaCl records the ratio of ions in the solid, rather than requiring isolated NaCl molecules throughout the crystal. 3 4
How to tell them apart
For a named substance, look up its accepted chemical identity. An element has an entry and symbol in the periodic table. A compound’s formula includes more than one element, although a formula alone may not show the complete arrangement of its atoms. 1 3
Capitalization matters: Co is cobalt, one element, while CO represents carbon monoxide, a compound of carbon and oxygen. A small typographic difference changes the chemical meaning. 1
For an actual sample, also establish purity. A label such as “iron” or “water” may name its main constituent without proving that the sample contains nothing else. Composition and structure, not color or a familiar product name, settle the classification. This follows from the distinction between substances and mixtures. 2
Where they overlap
Elements and compounds are the two major classes of pure substances in introductory chemistry. Compounds are built from elements, so detecting the element oxygen in water is consistent with water being a compound. The statement describes a constituent, not a competing identity for the entire substance. 2
Both categories also include substances in different physical states. Being a gas, liquid, or solid does not decide whether something is elemental or compound. 2
Edge cases
Molecular elements remain elements. O₂ and S₈ contain several atoms per molecule, but only one element each. A molecule does not have to be a compound. 3
Different isotopes remain the same element. Their neutron counts differ while their proton counts match. “One kind of atom” is useful shorthand only if kind means elemental identity. 1
Not every compound is a collection of small molecules. Ionic crystals and covalent network solids require descriptions of extended structures. A water-molecule drawing is not a universal model of compounds. 4
Why the distinction exists
Chemistry needs to distinguish elemental identity from the substances formed when different elements combine. The distinction explains why ordinary chemical reactions rearrange constituents into new substances while elemental labels still allow us to track what is present. Atomic number makes that identity precise. 1 2
Common misconceptions
Examples
Elemental iron is an element in substance form. Oxygen gas, O₂, is an elemental substance made of molecules. Pure water, H₂O, is a compound made of molecules. A saltwater sample is a mixture. The examples deliberately separate elemental identity, molecular structure, and sample composition. 2 3
Sources
Sources checked October 3, 2026.
- OpenStax, Chemistry 2e — Atomic Structure and Symbolism. Atomic number, isotopes, ions, and symbol capitalization.
- OpenStax, Chemistry 2e — Phases and Classification of Matter. Elements, compounds, and mixtures.
- OpenStax, Chemistry 2e — Chemical Formulas. Molecular elements, compound formulas, and ratios.
- OpenStax, Chemistry 2e — The Solid State of Matter. Ionic and covalent network solids.