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1) In chemistry, a compound is a substance that results from a combination of two or more different chemical element s, in such a way that the atom s of the different elements are held together by chemical bonds that are difficult to break. These bonds form as a result of the sharing or exchange of electron s among the atoms. The smallest unbreakable unit of a compound is called a molecule .

A compound differs from a mixture, in which bonding among the atoms of the constituent substances does not occur. In some situations, different elements react with each other when they are mixed, forming bonds among the atoms and thereby producing molecules of a compound. In other scenarios, different elements can be mixed and no reaction occurs, so the elements retain their individual identities. Sometimes, when elements are mixed, the reaction occurs slowly (as when iron is exposed to oxygen); in other cases it takes place rapidly (as when lithium is exposed to oxygen). Sometimes, when an element is exposed to a compound, a reaction occurs in which new compounds are formed (as when pure elemental sodium is immersed in liquid water).

Often, a compound looks and behaves nothing like any of the elements that comprise it. Consider, for example, hydrogen (H) and oxygen (O). Both of these elements are gases at room temperature and normal atmospheric pressure. But when they combine into the familiar compound known as water, each molecule of which contains two hydrogen atoms and one oxygen atom (H 2 O), the resulting substance is a liquid at room temperature and normal atmospheric pressure.

The atoms of a few elements do not readily bond with other elements to form compounds. These are called noble or inert gases: helium, neon, argon, krypton, xenon, and radon. Certain elements readily combine with other elements to form compounds. Examples are oxygen, chlorine, and fluorine.

2) As an adjective, compound is the quality of being composed of multiple parts. Examples of this usage include compound eye (found in various insects), compound microscope (a high-power magnifying instrument consisting of more than one lens), compound sentence (a sentence that is a conjunction or disjunction of smaller sentences), and compound document , an organized collection of user interfaces that form a single integrated perceptual environment.

This was last updated in September 2005
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