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 (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.


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 formed from atoms.

In the following diagram:

Atoms are represented by single spheres.
Spheres of the same size and colour represent atoms of the same element.

Molecules are represented by two or more spheres joined together.

Molecules of Elements would be represented by two or more spheres of the same size and colour joined together.

Molecules of Compounds are represented by two or more spheres of different sizes and colours joined together, as shown below.

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

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