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A molecule is the smallest portion of a pure element or compound that retains a set of
unique chemical and physical properties. A molecule usually consists of two or more atoms
bonded together, with the exception of noble gases which are composed of single atom molecules.
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
(either ionic or covalent bonding) connecting the entire structure. Another example of a macroscopically-sized molecule is
the DNA, a macromolecule.
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.
Chemical formula on the other hand reflects the exact number of
atoms that compose a molecule. The molecular mass is calculated from
the chemical formula and is expressed in conventional units equal to 1/12 from the mass of a 12C isotope atom.
See also: Polar and nonpolar molecules.
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