text: Symbols for chemical elements shall be written in roman (upright) type. The symbol is not followed by a full-stop.
  Examples:
  : H He C Ca
  Attached subscripts or superscripts specifying a nucleotide or molecule have the following meanings and positions:
  * The nucleon number (mass number) is shown in the left superscript position (e.g., N)
  * The number of atoms of a nucleotide is shown in the right subscript position (e.g., N)
  * The proton number (atomic number) may be indicated in the left subscript position (e.g., Gd)
  * If necessary, a state of ionization or an excited state may be indicated in the right superscript position (e.g., state of ionization Na)