Editing Chemical element (section)

=== Properties ===
Several kinds of descriptive categorisations can be applied broadly to the elements, including consideration of their general physical and chemical properties, their states of matter under familiar conditions, their melting and boiling points, their densities, their crystal structures as solids, and their origins.

==== General properties ====
Several terms are commonly used to characterise the general physical and chemical properties of the chemical elements. A first distinction is between [[metal]]s, which readily conduct [[electricity]], [[nonmetal]]s, which do not, and a small group, (the ''[[metalloid]]s''), having intermediate properties and often behaving as [[semiconductor]]s.

A more refined classification is often shown in coloured presentations of the periodic table. This system restricts the terms "metal" and "nonmetal" to only certain of the more broadly defined metals and nonmetals, adding additional terms for certain sets of the more broadly viewed metals and nonmetals. The version of this classification used in the periodic tables presented here includes: [[actinide]]s, [[alkali metal]]s, [[alkaline earth metal]]s, [[halogen]]s, [[lanthanide]]s, [[transition metal]]s, [[post-transition metal]]s, [[metalloid]]s, [[reactive nonmetal]]s, and [[noble gas]]es. In this system, the alkali metals, alkaline earth metals, and transition metals, as well as the lanthanides and the actinides, are special groups of the metals viewed in a broader sense. Similarly, the reactive nonmetals and the noble gases are nonmetals viewed in the broader sense. In some presentations, the halogens are not distinguished, with [[astatine]] identified as a metalloid and the others identified as nonmetals.

==== States of matter ====
Another commonly used basic distinction among the elements is their [[state of matter]] (phase), whether [[solid]], [[liquid]], or [[gas]], at [[standard temperature and pressure]] (STP). Most elements are solids at STP, while several are gases. Only [[bromine]] and [[mercury (element)|mercury]] are liquid at 0 degrees Celsius (32 degrees Fahrenheit) and 1 atmosphere pressure; [[caesium]] and [[gallium]] are solid at that temperature, but melt at 28.4°C (83.2°F) and 29.8°C (85.6°F), respectively.

==== Melting and boiling points ====
[[Melting point|Melting]] and [[boiling point]]s, typically expressed in degrees [[Celsius]] at a pressure of one atmosphere, are commonly used in characterizing the various elements. While known for most elements, either or both of these measurements is still undetermined for some of the radioactive elements available in only tiny quantities. Since helium remains a liquid even at [[absolute zero]] at atmospheric pressure, it has only a boiling point, and not a melting point, in conventional presentations.

==== Densities ====
{{Main|Densities of the elements (data page)}}

The [[density]] at selected [[standard temperature and pressure]] (STP) is often used in characterizing the elements. Density is often expressed in grams per cubic centimetre (g/cm{{sup|3}}). Since several elements are gases at commonly encountered temperatures, their densities are usually stated for their gaseous forms; when liquefied or solidified, the gaseous elements have densities similar to those of the other elements.

When an element has allotropes with different densities, one representative allotrope is typically selected in summary presentations, while densities for each allotrope can be stated where more detail is provided. For example, the three familiar [[allotropes of carbon]] ([[amorphous carbon]], [[graphite]], and [[diamond]]) have densities of 1.8–2.1, 2.267, and 3.515&nbsp;g/cm{{sup|3}}, respectively.

==== Crystal structures ====
{{Main|Crystal structure}}

The elements studied to date as solid samples have eight kinds of [[crystal structure]]s: [[cubic crystal system|cubic]], [[cubic crystal system|body-centered cubic]], face-centered cubic, [[Hexagonal crystal system|hexagonal]], [[Monoclinic crystal system|monoclinic]], [[orthorhombic crystal system|orthorhombic]], [[Trigonal crystal system|rhombohedral]], and [[Tetragonal crystal system|tetragonal]]. For some of the synthetically produced transuranic elements, available samples have been too small to determine crystal structures.

==== Occurrence and origin on Earth ====
{{Main|Abundance of elements in Earth's crust}}
Chemical elements may also be categorised by their origin on Earth, with the first 94 considered naturally occurring, while those with atomic numbers beyond 94 have only been produced artificially via human-made nuclear reactions.

Of the 94 naturally occurring elements, 83 are considered primordial and either [[stable isotope|stable]] or weakly radioactive. The longest-lived isotopes of the remaining 11 elements have [[Half-life|half lives]] too short for them to have been present at the beginning of the Solar System, and are therefore "transient elements". Of these 11 transient elements, five ([[polonium]], [[radon]], [[radium]], [[actinium]], and [[protactinium]]) are relatively common [[decay product]]s of [[thorium]] and [[uranium]]. The remaining six transient elements (technetium, promethium, astatine, [[francium]], [[neptunium]], and [[plutonium]]) occur only rarely, as products of rare decay modes or nuclear reaction processes involving uranium or other heavy elements.

Elements with atomic numbers 1 through 82, except 43 (technetium) and 61 (promethium), each have at least one isotope for which no radioactive decay has been observed. Observationally stable isotopes of some elements (such as [[tungsten]] and [[lead]]), however, are predicted to be slightly radioactive with very long half-lives:{{NUBASE2016|ref}} for example, the half-lives predicted for the observationally stable lead isotopes range from 10{{sup|35}} to 10{{sup|189}} years. Elements with atomic numbers 43, 61, and 83 through 94 are unstable enough that their radioactive decay can be detected. Three of these elements, bismuth (element 83), thorium (90), and uranium (92) have one or more isotopes with half-lives long enough to survive as remnants of the explosive [[stellar nucleosynthesis]] that produced the heavy elements before the formation of the Solar System. For example, at over 1.9{{e|19}} years, over a billion times longer than the estimated age of the universe, [[bismuth-209]] has the longest known [[alpha decay]] half-life of any isotope.{{r|Dume2003}}{{r|Marcillac2003}} The last 24 elements (those beyond plutonium, element 94) undergo radioactive decay with short half-lives and cannot be produced as daughters of longer-lived elements, and thus are not known to occur in nature at all.