Inchemistry and atomicphysics, themain group is thegroup ofelements (sometimes called therepresentative elements) whose lightest members are represented byhelium,lithium,beryllium,boron,carbon,nitrogen,oxygen, andfluorine as arranged in theperiodic table of the elements. The main group includes the elements (excepthydrogen, which is sometimes not included[citation needed]) in groups 1 and 2 (s-block), and groups 13 to 18 (p-block). The s-block elements are primarily characterised by one main oxidation state, and the p-block elements, when they have multiple oxidation states, often have common oxidation states separated by two units.
Main-group elements (with some of the lightertransition metals) are the mostabundant elements onEarth, in theSolar System, and in theuniverse.Group 12 elements are often considered to be transition metals; however,zinc (Zn),cadmium (Cd), andmercury (Hg) share some properties of both groups, and some scientists believe they should be included in the main group.[1][2]
Occasionally, even thegroup 3 elements as well as thelanthanides andactinides have been included, because especially the group 3 elements and many lanthanides areelectropositive 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) anduranium (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).[3]
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).
