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

1 Answer
Jul 22, 2017

Answer:

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.

Periodic Table
(From Alice in Galaxyland)