What is a Compound ?

Matter can be classified in various ways according to its structure, behaviours and physical and chemical properties.

The main classifications of matter include the categories element and compound, either of which may also be called a "substance" (which is a less specific term), and mixture - of which there are also many sub-categories.

Definitions of a Chemical Compound:

A compound is the result of two or more elements becoming chemically combined together. The elements react chemically, causing bonds to be formed between the atoms involved ... resulting in molecules of the compound. The elements react in specific amounts, resulting in a compound that has a specific chemical composition. Compounds do not necessarily resemble the elements of which they are composed and cannot be converted back to those elements by physical methods. It can even be very difficult and require a lot of energy to change compounds into their elements by chemical means (reactions).

A compound is a pure substance that consists of two or more elements chemically combined in a fixed proportion, that can be further subdivided into simpler substances by chemical (not physical) means only. ... So, if a quantity of a material consists of atoms of two or more elements joined together, always in the same ratio, then the matter forming that material is a compound.

List of facts about Compounds:

1. Compounds consist of molecules formed from atoms of 2 or more different elements bound together chemically.
2. Compounds can be broken down into a simpler type of matter (elements) by chemical means; not by physical means
3. Compounds always contains the same ratio of component elements.
4. Compounds have properties different from their component elements
   e.g. the compound water (H₂O) is a liquid at room temperature and pressure and has different chemical properties from those of the two elements, hydrogen (H₂) and oxygen (O₂), from which it is formed.
5. Compounds can be represented using chemical formulae.

So,

A compound can be represented by a chemical formula.

A molecule is the smallest part of a compound whose properties are those of the compound.

Do compounds consist of atoms or molecules ?

Compounds consist of molecules - which are, in turn, formed from atoms.

See also the page about Elements Mixtures and Compounds, and Atoms and Molecules.

Note: This is one of a series of simple pages introducing key concepts in introductory chemistry. Other pages in this section include elements, mixtures and compounds and individual pages about substances, elements, mixtures and compounds, plus pages about atoms, molecules and isotopes. If you need further information ask your chemistry tutor.