The formula for the chemical component. Formulae are written
according to the following rules:
(1) Only recognized element symbols may be used.
(2) Each element symbol is followed by a 'count' number. A count
of '1' may be omitted.
(3) A space or parenthesis must separate each cluster of
(element symbol + count), but in general parentheses are
(4) The order of elements depends on whether carbon is
present or not. If carbon is present, the order should be:
C, then H, then the other elements in alphabetical order
of their symbol. If carbon is not present, the elements
are listed purely in alphabetic order of their symbol. This
is the 'Hill' system used by Chemical Abstracts.