Column of elements in the periodic table of the chemical elements
In theperiodic table of the elements, each column is agroup.
Inchemistry, agroup (also known as afamily)[1] is a column of elements in theperiodic table of the chemical elements. There are 18 numbered groups in the periodic table; the 14f-block columns, between groups 2 and 3, are not numbered. The elements in a group have similar physical or chemical characteristics of the outermostelectron shells of their atoms (i.e., the samecore charge), because most chemical properties are dominated by the orbital location of the outermost electron.
The modern numbering system of "group 1" to "group 18" has been recommended by theInternational Union of Pure and Applied Chemistry (IUPAC) since 1988.[citation needed] The 1-18 system is based on each atom's s, p and d electrons beyond those in atoms of the precedingnoble gas. Two older incompatible naming schemes can assign the same number to different groups depending on the system being used. The older schemes were used by theChemical Abstract Service (CAS, more popular in the United States), and by IUPAC before 1988 (more popular in Europe). The system of eighteen groups is generally accepted by the chemistry community, but some dissent exists about membership ofelements number 1 and 2 (hydrogen andhelium). Similar variation on theinner transition metals continues to exist in textbooks, although the correct positioning has been known since 1948 and was twice endorsed by IUPAC in 1988 (together with the 1–18 numbering) and 2021.
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 togroup 8, but in chemistry may also meaniron,cobalt, andnickel, or some other set of elements with similar chemical properties. Inastrophysics andnuclear physics, it usually refers to iron, cobalt, nickel,chromium, andmanganese.
Modern group names are numbers 1–18, with the 14 f-block columns remaining unnumbered (together making the32 columns in the periodic table). Also, trivial names (likehalogens) are common. In history, several sets of group names have been used, based on Roman numberings I–VIII, and "A" and "B" suffixes.[2][3]
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 14f-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[4] and 2021[5] 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,[6] andLev Landau andEvgeny Lifshitz already considered it incorrect in 1948.[7] Arguments can still occasionally be encountered in the contemporary literature purporting to defend it, but most authors consider them logically inconsistent.[8][9][10] 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.[5] See alsoGroup 3 element#Composition. d Group 18, the noble gases, had not been 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.
^fCoinage metals: authors differ on whether roentgenium (Rg) is considered a coinage metal. It is in group 11, like the other coinage metals, and is expected to be chemically similar to gold.[17] On the other hand, being extremely radioactive and short-lived, it cannot actually be used for coinage as the name suggests, and on that basis it is sometimes excluded.[18]
^btriels (group 13), from Greektri: three, III[13][16]
^ctetrels (group 14), from Greektetra: four, IV[13][16]
Two earlier group number systems exist:CAS (Chemical Abstracts Service) andold IUPAC. Both usenumerals (Arabic orRoman) and lettersA andB. 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 (seeList 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 onevalence 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 tomain 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,[2] and was later included as part of the 1990 edition of theNomenclature of Inorganic Chemistry.[19]
^William B. Jensen (1982). "The Positions of Lanthanum (Actinium) and Lutetium (Lawrencium) in the Periodic Table".J. Chem. Educ.59 (8):634–636.Bibcode:1982JChEd..59..634J.doi:10.1021/ed059p634.
^Chemey, Alexander T.; Albrecht-Schmitt, Thomas E. (2019). "Evolution of the periodic table through the synthesis of new elements".Radiochimica Acta.107 (9–11):1–31.doi:10.1515/ract-2018-3082.
^Conradie, Jeanet; Ghosh, Abhik (2019). "Theoretical Search for the Highest Valence States of the Coinage Metals: Roentgenium Heptafluoride May Exist".Inorganic Chemistry.58 (13):8735–8738.doi:10.1021/acs.inorgchem.9b01139.PMID31203606.S2CID189944098.
