Group (periodic table)


In chemistry, a group 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 are not numbered. The elements in a group have similar physical or chemical characteristics of the outermost electron shells of their atoms, 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 since about 1990. It replaces two older incompatible naming schemes, used by the Chemical Abstract Service, and by IUPAC before 1990. 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, 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.

Group names

In history, several sets of group names have been used:
New
IUPAC
name		Old
IUPAC
CAS
name
Name
by element		IUPAC
recommended
trivial name		Other trivial name

lithium family		hydrogen
and
alkali metals*
beryllium familyalkaline earth metals*
scandium family
titanium family
vanadium family
chromium family
manganese family
iron family
cobalt family
nickel family
copper familycoinage metals
zinc family
boron familytriels from Greek tri
carbon familytetrels from Greek tetra
nitrogen familypnictogens*pentels from Greek penta
oxygen familychalcogens*
fluorine familyhalogens*
family
or 		noble gases*

Some other names have been proposed and used without gaining wide acceptance: "volatile metals" for group 12; "icosagens" for group 13; "crystallogens", "adamantogens", and "merylides" for group 14; and "aerogens" for group 18.

CAS and old IUPAC numbering (A/B)

Two earlier group number systems exist: CAS and old IUPAC. Both use numerals and letters A and B. Both systems agree on the numbers. The numbers indicate approximately the highest oxidation number of the elements in that group, and so indicate similar chemistry with other elements with the same numeral. The number proceeds in a linearly increasing fashion for the most part, once on the left of the table, and once on the right, with some irregularities in the transition metals. However, the two systems use the letters differently. For example, potassium has one valence electron. Therefore, it is located in group 1. Calcium is in group 2, for it contains two valence electrons.
In the old IUPAC system the letters A and B were designated to the left and right part of the table, while in the CAS system the letters A and B are designated to main group elements and transition elements. The old IUPAC system was frequently used in Europe, while the CAS is most common in America. The new IUPAC scheme was developed to replace both systems as they confusingly used the same names to mean different things. The new system simply numbers the groups increasingly from left to right on the standard periodic table. The IUPAC proposal was first circulated in 1985 for public comments, and was later included as part of the 1990 edition of the Nomenclature of Inorganic Chemistry.