Appendix:Chemical elements/English

Periodic table
  For more information, see. Note that classification is not always agreed between sources (e.g. the set of metalloids) and classes can overlap (e.g. astatine is likely both a metal and a halogen). The elements La–Yb and Ac–No belong between groups 2 and 3, but are usually put below for reasons of space (not making the table too wide). Both positionings are nonetheless acceptable and used. The group numbers and placements shown are in accordance with reports of the International Union of Pure and Applied Chemistry from 1988 and 2021.

Older periodic tables may show different formats, e.g. Mendeleev's system with only 8 groups. Many other formats have also been suggested, though with few exceptions (e.g. Janet's left-step table, Bayley's pyramid, or Benfey's spiral), they have not gotten much takeup, and none of them threaten the primacy of the standard form. Even within the modern form, today the f-block is sometimes still moved one element to the right, so that group 3 becomes Sc-Y-La-Ac and the f-block contains Ce–Lu and Th–Lr. (This form comes from initial false expectations and measurements of the electron configurations of the elements and was already considered to be incorrect in 1948 by Soviet physicists Lev Landau and Evgeny Lifshitz.)

Discussion still continues about the placements of hydrogen and helium (the first row): electronically (and somewhat chemically), hydrogen belongs in group 1 (though some authors prefer to put it in group 17 or place it floating outside all groups), but the criteria are in clear conflict for helium and there is significant discussion in modern chemical literature on whether it should be moved to group 2 (taking electronics as primary) or remain in group 18 (taking chemistry as primary).

Ordered list
Elements beyond 118 have not yet been discovered: such elements are called by a temporary IUPAC, concatenating Greek and Latin roots before the usual suffix "-ium". (This suffix overrides the traditional chemical suffixes, so "-ium" is used even if the element would be a halogen or noble gas.) After an element is discovered, it keeps the systematic name until its discovery meets the criteria of and is accepted by the IUPAC/IUPAP Joint Working Party, upon which the discoverers are invited to propose a permanent name and symbol. Once this is announced, a comment period then follows until the permanent name and symbol become official and displace the systematic ones.

Prefix forms
Many, but not all, elements have an associated prefix form.