The formula for the reference entity. Formulae are written
according to the rules:
1. Only recognised 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 element symbol and
its count, but in general parentheses are not used.
4. The order of elements depends on whether or not carbon is
present. 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.