In
chemistry, a
group (also known as a
family) is a column of elements in the
periodic table of the
chemical elements. There are 18 numbered groups in the periodic table, but the
f-block columns (between groups 2 and 3) are not numbered. The elements in a group have similar physical or chemical characteristics of the outermost
electron shells of their atoms (i.e., the same
core charge), as most chemical properties are dominated by the orbital location of the outermost electron. There are three systems of group numbering. The modern numbering
group 1 to
group 18 is recommended by the
International Union of Pure and Applied Chemistry (IUPAC). It replaces two older naming schemes that were mutually confusing. Also, groups may be identified by their topmost element or have a specific name. For example, group 16 is variously described as
oxygen group and
chalcogen.