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In chemistry, the molecule is the smallest indivisible portion of a pure compound that retains a set of unique chemical and physical properties. A molecule consists of two or more atoms bonded together.

Figure 1, 3D (left and center) and 2D (right) representations of the terpenoid, atisane[?]. In the 3D model on the left, carbon atoms are represented by gray spheres, white spheres represent the hydrogen atoms and the cylinders represent the bonds. The model is enveloped in a "mesh" representation of the molecular surface, colored by areas of positive (red) and negative (blue) charge. In the 3D model (center), the light-blue spheres represent carbon atoms, the white spheres are hydrogen atoms, and the cylinders in between the atoms correspond to single-bonds.

Most molecules are much too small to be seen with the naked eye, but there are exceptions. A grain of salt, or the diamond on an engagement ring, are giant crystal lattices, repetitive molecules with atomic bonding (usually ionic bonding) connecting the entire structure.

A property of molecules is the integer ratio of the elements that constitute the compound, the empirical formula. For example, in their pure forms, water is always composed of a 2:1 ratio of hydrogen to oxygen, and ethyl alcohol or ethanol is always composed of carbon, hydrogen, and oxygen in a 2:6:1 ratio. However, this does not determine the kind of molecule uniquely - dimethyl ether[?] has the same ratio as ethanol, for instance. Molecules with the same atoms in different arrangements are called isomers.

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