In chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two atoms in a definite
arrangement held together by very strong chemical bonds. It can also be defined as a unit of two or more atoms held
together by covalent bonds. Molecules are distinguished from polyatomic ions in this strict sense. In organic chemistry and
biochemistry, the term molecule is used less strictly and also is applied to charged organic molecules and biomolecules.

This definition has evolved as knowledge of the structure of molecules has increased. Earlier definitions were less precise
defining molecules as the smallest particles of pure chemical substances that still retain their composition and chemical
properties.[3] This definition often breaks down since many substances in ordinary experience, such as rocks, salts, and
metals, are composed of large networks of chemically bonded atoms or ions, but are not made of discrete molecules.

In the kinetic theory of gases the term molecule is often used for any gaseous particle regardless of their composition.[4]
According to this definition noble gas particles are considered molecules despite the fact that they are composed of a
single non-bonded atom.

A molecule may consist of atoms of the same chemical element, as with oxygen (O2), or of different elements, as with
water (H2O). Atoms and complexes connected by non-covalent bonds such as hydrogen bonds or ionic bonds are
generally not considered single molecules.

No typical molecule can be defined for ionic crystals (salts) and covalent crystals (network solids), although these are often
composed of repeating unit cells that extend either in a plane (such as in graphite) or three-dimensionally (such as in
diamond or sodium chloride). The theme of repeated unit-cellular-structure also holds for most condensed phases with
metallic bonding. In glasses (solids that exist in a vitreous disordered state), atoms may also be held together by chemical
bonds without any definable molecule, but also without any of the regularity of repeating units that characterises crystals.

Source: WikepediA
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