Group (periodic table)

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, but the f-block columns (between groups 2 and 3) are not numbered. The elements in a group have similar physical or chemical characteristic 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.

CAS and old IUPAC numbering

Two earlier group number systems exist: CAS (Chemical Abstracts Service) and old IUPAC. Both use numerals (Arabic or Roman) 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 (see List of oxidation states of the elements), with some irregularities in the transition metals. However, the two systems use the letters differently. For example, potassium (K) has one valence electron. Therefore, it is located in group 1. Calcium (Ca) 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 (A) and right (B) part of the table, while in the CAS system the letters A and B are designated to main group elements (A) and transition elements (B). 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.

Group names

In history, several sets of group names have been used:

Groups in the periodic table

IUPAC group	1a	2	—b	3c	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
Mendeleev (I–VIII)	IA	IIA		IIIB	IVB	VB	VIB	VIIB	VIIIB			IB	IIB	IIIB	IVB	VB	VIB	VIIB	d
CAS (US, A-B-A)	IA	IIA		IIIB	IVB	VB	VIB	VIIB	VIIIB			IB	IIB	IIIA	IVA	VA	VIA	VIIA	VIIIA
Old IUPAC (Europe, A-B)	IA	IIA		IIIA	IVA	VA	VIA	VIIA	VIIIB			IB	IIB	IIIB	IVB	VB	VIB	VIIB	0
Trivial namer	H and alkali metals	alkaline earth metals												triels	tetrels	pnicto­gens	chal­co­gens	halo­gens	noble gases
Name by elementr	lith­ium group	beryl­lium group		scan­dium group	titan­ium group	vana­dium group	chro­mium group	man­ga­nese group	iron group	co­balt group	nickel group	cop­per group	zinc group	boron group	car­bon group	nitro­gen group	oxy­gen group	fluor­ine group	helium or neon group
Period 1	H																		He
Period 2	Li	Be												B	C	N	O	F	Ne
Period 3	Na	Mg												Al	Si	P	S	Cl	Ar
Period 4	K	Ca		Sc	Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn	Ga	Ge	As	Se	Br	Kr
Period 5	Rb	Sr		Y	Zr	Nb	Mo	Tc	Ru	Rh	Pd	Ag	Cd	In	Sn	Sb	Te	I	Xe
Period 6	Cs	Ba	La–Yb	Lu	Hf	Ta	W	Re	Os	Ir	Pt	Au	Hg	Tl	Pb	Bi	Po	At	Rn
Period 7	Fr	Ra	Ac–No	Lr	Rf	Db	Sg	Bh	Hs	Mt	Ds	Rg	Cn	Nh	Fl	Mc	Lv	Ts	Og

^a Group 1 is composed of hydrogen (H) and the alkali metals. Elements of the group have one s-electron in the outer electron shell. Hydrogen is not considered to be an alkali metal as it is not a metal, though it is more analogous to them than any other group. This makes the group somewhat exceptional.
^b The 14 f-block groups (columns) do not have a group number.
^c The correct composition of group 3 is scandium (Sc), yttrium (Y), lutetium (Lu), and lawrencium (Lr), as shown here: this is endorsed by 1988 and 2021 IUPAC reports on the question. General inorganic chemistry texts often put scandium (Sc), yttrium (Y), lanthanum (La), and actinium (Ac) in group 3, so that Ce–Lu and Th–Lr become the f-block between groups 3 and 4; this was based on incorrectly measured electron configurations from history, and Lev Landau and Evgeny Lifshitz already considered it incorrect in 1948. Arguments can still occasionally be encountered in the contemporary literature purporting to defend it, but most authors consider them logically inconsistent. Some sources follow a compromise that puts La–Lu and Ac–Lr as the f-block rows (despite that giving 15 f-block elements in each row, which contradicts quantum mechanics), leaving the heavier members of group 3 ambiguous. See also Group 3 element#Composition.
^d Group 18, the noble gases, were not discovered at the time of Mendeleev's original table. Later (1902), Mendeleev accepted the evidence for their existence, and they could be placed in a new "group 0", consistently and without breaking the periodic table principle.
^r Group name as recommended by IUPAC.

New IUPAC names	Old IUPAC (Europe)	CAS (U.S.)	Name by element	Trivial name (*=IUPAC approved)	note
Group 1	IA	IA	lithium family	alkali metals*	Hydrogen is in this column, but not considered an alkali metal
Group 2	IIA	IIA	beryllium family	alkaline earth metals*
Group 3	IIIA	IIIB	scandium family		Consisting of rare earth elements plus actinides
Group 4	IVA	IVB	titanium family
Group 5	VA	VB	vanadium family
Group 6	VIA	VIB	chromium family
Group 7	VIIA	VIIB	manganese family
Group 8	VIII	VIIIB	iron family
Group 9	VIII	VIIIB	cobalt family
Group 10	VIII	VIIIB	nickel family
Group 11	IB	IB	copper family	coinage metals
Group 12	IIB	IIB	zinc family	volatile metals
Group 13	IIIB	IIIA	boron family	triels, icosagens	triels from Greek tri (three, III)
Group 14	IVB	IVA	carbon family	tetrels, crystallogens	tetrels from Greek tetra (four, IV)
Group 15	VB	VA	nitrogen family	pentels, pnictogens*	pentels from Greek penta (five, V)
Group 16	VIB	VIA	oxygen family	chalcogens*
Group 17	VIIB	VIIA	fluorine family	halogens*
Group 18	Group 0	VIIIA	helium family or neon family	noble gases*, aerogens

Further reading

• Scerri, E. R. (2007). The periodic table, its story and its significance. Oxford University Press. ISBN 978-0-19-530573-9. {{cite book}}: line feed character in |title= at position 30 (help)