in the periodic table
of the elements, each numbered column is a group
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; the f-block columns (between groups 3 and 4) 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), because most chemical properties are dominated by the orbital location of the outermost electron.
there are three systems of group numbering for the groups; the same number may be assigned to different groups depending on the system being used. the modern numbering system of "group 1" to "group 18" has been recommended by the international union of pure and applied chemistry (iupac) since about 1990. it replaces two older incompatible naming schemes, used by the chemical abstract service (cas, more popular in the us), and by iupac before 1990 (more popular in europe). the system of eighteen groups is generally accepted by the chemistry community, but some dissent exists about membership of several elements. disagreements mostly involve elements number 1 and 2 (hydrogen and helium), as well as inner transition metals.
groups may also be identified using their topmost element, or have a specific name. for example, group 16 is also described as the "oxygen group" and as the "chalcogens". an exception is the "iron group", which usually refers to "group 8", but in chemistry may also mean iron, cobalt, and nickel, or some other set of elements with similar chemical properties. in astrophysics and nuclear physics, it usually refers to iron, cobalt, nickel, chromium, and manganese.