Why the entire d block starts with group IIIB and not IB (these are placed right after VIIIB)?
The numbers indicate approximately the highest oxidation number of the elements in each group.
The numbering is an old system called the CAS System (Chemical Abstracts Services System).
The main-group elements (Groups 1,2, and 13 to 18) were assigned to the A series, and the
The numbers indicated approximately the highest oxidation number of the elements in each group.
Thus, the assignments were:
Here's an example of a Periodic Table that uses both the CAS system and the new IUPAC numbering.
(From Alice in Galaxyland)