<P> Group 12 elements are often considered to be transition metals; however, zinc (Zn), cadmium (Cd), and mercury (Hg) share some properties of both groups, and many scientists believe they should be included in the main group . Occasionally, even the group 3 elements as well as the lanthanides and actinides have been included, because especially the group 3 elements and lanthanides are electropositive elements with only one main oxidation state like the group 1 and 2 elements . The position of the actinides is more questionable, but the most common and stable of them, thorium (Th) and uranium (U), are similar to main - group elements as thorium is an electropositive element with only one main oxidation state (+ 4), and uranium has two main ones separated by two oxidation units (+ 4 and + 6). </P> <P> In older nomenclature the main - group elements are groups IA and IIA, and groups IIIB to 0 (CAS groups IIIA to VIIIA). Group 12 is labelled as group IIB in both systems . Group 3 is labelled as group IIIA in the older nomenclature (CAS group IIIB). </P> <P> Main - group elements (with some of the lighter transition metals) are the most abundant elements on Earth, in the Solar System, and in the Universe . They are sometimes also called the representative elements . </P>

What category of elements is the most common in the periodic table