The numeration is the way of numbering the 18 groups of the periodic table. You can change the numeration to IUPAC, old IUPAC or CAS, or you can switch it off entirely.

The following options in the ViewNumeration menu can be used to change the numeration shown:

  • No Numeration: if this option is active, no period-numeration will be in effect.

  • IUPAC (default) is the International Union of Pure and Applied Chemistry. This is an organization which defines most of the standards for chemical concerns. The new IUPAC system numbers each column with Arabic numbers from 1 (one) through 18 (eighteen).

  • CAS is the Chemical Abstracts Service. In the CAS system the letters A and B were designated to main group elements (A) and transition elements (B). Though the IUPAC numeration is the official, the CAS numeration is what is still used in classrooms and laboratories.

  • The Old IUPAC system labeled columns with Roman numerals followed by either the letter A or B. Columns were numbered such that columns one through seven were numbered IA through VIIA, columns 8 through 10 were labeled VIIIA, columns 11 through 17 were numbered IB through VIIB and column 18 was numbered VIII. Because of the confusion the old IUPAC and the CAS system created, the IUPAC adopted their new system.