Help:Periodic table classifications in the English Wikipedia

The periodic table orders the chemical elements by their electronic properties, showing several patterns in chemical and physical properties. Some details that vary are arbitrary: they do not affect the scientific substance (like whether or not to cut out the f-block). Also, editorial choices can be made, such as the use of colors to use. Others depend on real scientific controversies.

This page describes the choices that were made in this English Wikipedia (enwiki), when showing the periodic table. Some are made in scientific disputes (like positioning of hydrogen), some are editorial (like background colors for metalloids).

Not all topics the periodic table shows are straightforward and undisputed. Some are chosen by looking at the scientific arguments presented in sources, which depend on chemistry and physics.

Apart from science, also editorial choices are made. For example, it is quite common to show the "18-column" format (with two extra rows below the main table), while the "32-column" form (the bottom rows put in their proper location) is showing exactly the same statements.

Periodic table
The basic periodic table in enwiki:

18-column and 32-column layout
These forms are only editorially (graphically) different, showing the same scientific statements. The above table shows the 18-column form: the one below shows the 32-column form.

The asterisks in the 18-column form make clear the theoretical location of the f-block elements La–Yb and Ac–No. As the forms are equivalent, editors should use whichever fits better in context.

IUPAC names
When available, enwiki uses names as defined by IUPAC. This is especially true for elements names aluminium, sulfur, caesium (which have variant spellings). So even when article aluminium is written in US-English, the US-spelling "aluminum" is not used.

When IUPAC has not provided or sanctioned a name, a common or descriptive name is used (metalloids, post-transition metals).

Group names
Groups (columns in the periodic table; sometimes called 'family') are numbered 1–18, following modern IUPAC guidelines. So called trivial group names, like halogens, are well defined and acceptable.

Each group can also be identified by its top element: group 14 = carbon group.

Earlier group naming systems use Roman numbers with an A- or B-suffix: VIA or VIB. They are described here. It is not advisable to use the earlier group-naming systems, because they are ambiguous: "group VIA" meant the oxygen group some systems, but meant the chromium group in others.


 * alkali metal = group 1 but without hydrogen (alkali is not strictly a group name then)
 * alkaline earth metal = group 2
 * pnictogen = group 15
 * chalcogen = group 16
 * halogen = group 17
 * noble gas = group 18
 * main-group element = s- and p-block elements
 * transition element = d-block element (groups 3 through 12)
 * inner transition element = f-block element
 * groups (columns) between group 2 and 3 are not numbered.

In many sources "transition metal" and "inner transition metal" are used instead of "transition element" and "inner transition element", as those elements are all metals.

The 2005 IUPAC Red Book excludes hydrogen from the main-group elements, but this exclusion is often not followed in practice, and the 2011 IUPAC Principles of Chemical Nomenclature does not exclude hydrogen.

The codification of those names is in the 2005 Red Book, when elements past Rg had not yet been approved. Per a 2016 IUPAC report "halogen" and "noble gas" include Ts and Og respectively. So, probably the intention is for "pnictogen" and "chalcogen" to include Mc and Lv respectively. That said, some sources talking about superheavy elements prefer to not call Ts and Og a halogen and a noble gas respectively (because they probably behave quite differently). So while including the whole group per IUPAC should be the default, it can make sense to clarify that this is being done.

The f-block columns do not have any IUPAC-approved group numbers. Neither do they have any generally recognised group numbers in common use. In general they are not often referred to (because they only have two elements each), but if a name is needed, they can be named by the first element (lanthanum group, cerium group, ..., ytterbium group).

Group 3: Sc/Y/Lu/Lr
This enwiki shows group 3 being Sc/Y/Lu/Lr, based on a IUPAC provisional report of January 2021. The discussion was conducted here. IUPAC had previously supported this format of group 3 in a 1988 report as well, and it shows that when displaying 32-column tables (then ambiguity is impossible).

Three formats of group 3 are common: one has Lu and Lr below Y (like the 1988 and 2021 IUPAC reports), one has La and Ac under Y, and one has blank spaces under Y. The controversy over group 3 in textbooks is such that no form has an absolute majority. However, a clear majority of writers publishing on the subject support the Sc/Y/Lu/Lr format of group 3, and they address the counterarguments offered by the (smaller) other sides. Combined with the IUPAC reports, this justifies the editorial decision chosen. It should also be noted that sources showing blank spaces under Y are quite sloppy about clarifying what that actually means, i.e. which elements are then being included in group 3.

Naturally, other formats of group 3 can be pictured when explaining the dispute.

Period 1
Enwiki shows hydrogen in group 1, and helium in group 18. So helium becomes the only s-block noble gas and has a different block from all the others. This is a popular format that agrees with the periodic tables drawn in those IUPAC reports mentioned above. Some sources have considered alternative placements (helium in group 2 following electronics is getting some attention recently), but IUPAC has not addressed those arguments, so we stick with the status quo.

Naturally, other forms of period 1 can be drawn when describing the dispute, or in drawing alternative periodic tables that take different positions. For example in Charles Janet's periodic table helium should certainly be in group 2 (because that is how Janet drew it).

Extended periodic table
Currently, 118 elements have been discovered and named, number 118 being oganesson. This happens to fill the whole periodic table in 7 rows (periods). Heavier elements (atomic number 119 and higher) are theoretically described, and start in period 8 (row 8). When these undiscovered elements are placed in the periodic table, it is called an extended periodic table.

The correct shape of an extended periodic table is under recent discussion in reliable sources. IUPAC has no opinion on the subject because it only considers the elements that are actually confirmed discoveries. Currently we show a likely outcome based on the most recent findings, but it may change with future research.

To refer to elements in the extension, either IUPAC systematic element names (e.g. ununennium, Uue) or atomic numbers (e.g. element 119, E119, 119) are OK.

Categories
Category is the word this Wikipedia uses for classifications in the metal-metalloid-nonmetal range. There is not any common word for it in the scientific world, and 'category' is not used for anything else.

In the past, various sets of categories were used to colour in the periodic table, most recently:


 * Metals
 * Alkali metal
 * Alkaline earth metal
 * Lanthanide
 * Actinide
 * Transition metal
 * Post-transition metal


 * Metalloid
 * Nonmetals
 * Reactive nonmetals
 * Noble gas


 * Unknown chemical properties
 * Category not known

Previously, the reactive nonmetals had been divided as "polyatomic nonmetals" and "diatomic nonmetals", or as "other nonmetals" and "halogens". But these schemes have all presented problems: oxygen is known in both polyatomic (ozone) and diatomic (dioxygen) forms; and while astatine is a halogen, it seems from recent experimental and theoretical evidence that it is probably not a nonmetal. Furthermore, there is significant variation in sources over what elements belong to what category. Most sources agree that B, Si, Ge, As, Sb, and Te are metalloids; but only a minority show precisely these six and no others.

Due to these neutrality problems, it was decided in 2021 to colour in blocks rather than categories. Blocks are defined according to which orbital is theoretically filling according to the Aufbau principle. Such a classification is the only one mentioned in the IUPAC Red Book (IR-3.5) that covers all elements. Nonetheless the categories can still be referred to, just without turning them into colour schemes.

The colours used for the blocks are:



The blocks beyond f are only theoretical as the corresponding elements have not been discovered yet (and for h and i, they might not even exist). But they can similarly be used (for consistency) as a colour-coding for the respective orbitals (which certainly exist).