Help:Periodic table classifications in the English Wikipedia
The periodic table orders the chemical elements, showing several patterns (periodicity), while also containing exceptions to those regularities.
Some topics in the periodic table are not straightforward and undisputed, but are chosen by scientific arguments (chemistry and physics). For example, the placement of hydrogen (H) can be in the left-upper corner (in group 1), or more to the right (in group 17). Apart from science, also editorial choices are made. For example, it is quite common to show the "18-column" format (with two extra rows moved to below the main table), while the 32-column form (the bottom rows put in their location) is showing exactly the same statements. Also, when elements are categorised as "noble gas", they have a key color (blue) which is a free editorial choice: being a noble gas does not enforce a particular color.
This page describes the choices that were made for the periodic table in the English Wikipedia (the one you are visiting right now). These choices, both scientific and editorial, are maintained throughout this wiki. The scientific choices are described in dedicated articles or sections, together with valid options that are not implemented. For example, the current set of metalloids is argued in the article, while also describing other possible sets.
Periodic table
18-column or 32-column layout
Equal
IUPAC names
When available, this wiki 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).
Groups
Group names
Groups (columns in the periodic table) are numbered 1–18, following modern IUPAC guidelines. Earlier group naming systems (Roman numbers with an A- or B-suffix: VIA or VIB), and trivial group names like halogens, are described here
Group 3: Sc/Y/La/Ac
Periods
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 element (atomic number 119 and higher) are theoretically described, and start in period 8 (row 8). When these elements are placed in the periodic table, it is called an extended periodic table.
Categories
Category is the word this Wikipedia uses for classifications in the metal-metalloid-nonmetal range. There is not common word for it in the scientific world, and 'category' is not used for anything else.
Categorising
The English Wikipedia categorises the elements in these categories:
The background colors are used as a legend, and are a local (Wiki) choice. Not all names are commonly used, and the inclusion may be arbitrary. For example, the list of metalloids varies between sources.
More categories
In special topics, more categories are needed. In these situations, the list of categories is extended with "(predicted)" colors (a lighter shade), superactinides and eka-superactinides.
History of changes
Under discussion: nonmetals categorised "Active nonmetals", "Less active nonmetals" and "noble gases" (2017)
Non-noble nonmetals currently are categorised "polyatomic nonmetals" and "diatomic nonmetals". Since March 2017, a discussion has taken place to change the categorisation for these nonmetals. Proposal is to use categories "Active nonmetals", "Less active nonmetals" and "Noble gases". (Noble gases unchanged).
Group 12: a post-transition metals, not transition metals (August 2017)
Group 12 (zinc, cadmium, mercury, copernicium) are calssified post-transition metal (not transition metal).
Heavy elements: unknown category (June 2017)
Of the heavy metals meitnerium (109Mt) to oganesson (118Og), only copernicium (112Cn) has a known category. The others are categorised "unknown".