The smallest unit of an element or compound
that can exist and still retain the
characteristics of the element or compound. A
molecule of an element consists of one or
more like atoms; a molecule of a compound
consists of two or more different atoms
bonded together. They vary in size and
complexity from the hydrogen molecule (H2) to
the large macromolecules found in polymers.
They are held together by electrovalent
bonds, in which the atoms gain or lose
electrons to form ions, or covalent bonds,
where electrons from each atom are shared in
a new molecular orbital. The symbolic
representation of a molecule is known as a
formula. The presence of more than one atom
is denoted by a subscript figure - for
example, one molecule of the compound water
is shown as H2O, having two atoms of hydrogen
and one atom of oxygen. According to the
molecular or kinetic theory of matter,
molecules are in a state of constant motion,
the extent of which depends on their
temperature, and exert forces on one another.
Molecules were inferrable from Avogadro's
hypothesis 1811, but only became generally
accepted in 1860 when proposed by Stanislao
Cannizzarro (1826-1910).