Question

Why the entire d block starts with group IIIB and not IB (these are placed right after VIIIB)?

Answer 1

The numbers indicate approximately the highest oxidation number of the elements in each group.

Explanation:

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 `"d"`-block elements were assigned to the B series.

The numbers indicated approximately the highest oxidation number of the elements in each group.

Thus, the assignments were:

`bbul(Groupcolor(white)(m) Assignment`
`color(white)(ll)"1 - 2"color(white)(mmmm)"IA-IIA"`
`color(white)(ll)"3 - 7"color(white)(mmmll)"IIIB-VIIB"`
`color(white)(ll)"8 - 10"color(white)(mmmm)"VIII"`
`"11 - 12"color(white)(mmmll)"IB-IIB"`
`"13 - 18"color(white)(mmm)"IIIA-VIIA"`

Here's an example of a Periodic Table that uses both the CAS system and the new IUPAC numbering.

(From Alice in Galaxyland)