Editing Periodic table (section)

=== Metallicity ===
[[File:Diamond cubic animation.gif|thumb|right|The diamond-cubic structure, a giant covalent structure adopted by carbon (as diamond), as well as by silicon, germanium, and (grey) tin, all in group 14.<br />(In grey tin, the band gap vanishes and metallization occurs.<ref>{{cite journal |last1=Carrasco |first1=Rigo A. |last2=Zamarripa |first2=Cesy M. |first3=Stefan |last3=Zollner |first4=José |last4=Menéndez |first5=Stephanie A. |last5=Chastang |first6=Jinsong |last6=Duan |first7=Gordon J. |last7=Grzybowski |first8=Bruce B. |last8=Claflin |first9=Arnold M. |last9=Kiefer |date=2018 |title=The direct bandgap of gray α-tin investigated by infrared ellipsometry |url=https://pubs.aip.org/aip/apl/article/113/23/232104/36404/The-direct-bandgap-of-gray-tin-investigated-by |journal=Applied Physics Letters |volume=113 |issue=23 |pages=232104 |doi=10.1063/1.5053884 |bibcode=2018ApPhL.113w2104C |s2cid=125130534 |access-date=|url-access=subscription }}</ref> Tin has another allotrope, white tin, whose structure is even more metallic.)]]
A simple substance is a substance formed from atoms of one chemical element. The simple substances of the more electronegative atoms tend to share electrons (form covalent bonds) with each other. They form either small molecules (like hydrogen or oxygen, whose atoms bond in pairs) or giant structures stretching indefinitely (like carbon or silicon). The noble gases simply stay as single atoms, as they already have a full shell.<ref name="cartoon" /> Substances composed of discrete molecules or single atoms are held together by weaker attractive forces between the molecules, such as the [[London dispersion force]]: as electrons move within the molecules, they create momentary imbalances of electrical charge, which induce similar imbalances on nearby molecules and create synchronized movements of electrons across many neighbouring molecules.<ref>{{cite web|url=https://www.chemguide.co.uk/atoms/bonding/vdw.html|title=Intermolecular bonding – van der Waals forces|access-date=17 November 2021|archive-date=22 January 2022|archive-url=https://web.archive.org/web/20220122154740/https://www.chemguide.co.uk/atoms/bonding/vdw.html|url-status=live}}</ref>

[[File:Graphite-and-diamond-with-scale.jpg|thumb|right|Graphite and diamond, two allotropes of carbon]]
The more electropositive atoms, however, tend to instead lose electrons, creating a "sea" of electrons engulfing cations.<ref name="cartoon" /> The outer orbitals of one atom overlap to share electrons with all its neighbours, creating a giant structure of molecular orbitals extending over all the atoms.<ref name="chemguidemetal">{{cite web |url=https://www.chemguide.co.uk/atoms/bonding/metallic.html |title=Metallic Bonding |last=Clark |first=Jim |date=2019 |website=Chemguide |access-date=30 March 2021 |archive-date=21 April 2021 |archive-url=https://web.archive.org/web/20210421105423/https://www.chemguide.co.uk/atoms/bonding/metallic.html |url-status=live }}</ref> This negatively charged "sea" pulls on all the ions and keeps them together in a [[metallic bond]]. Elements forming such bonds are often called [[metal]]s; those which do not are often called [[Nonmetal (chemistry)|nonmetal]]s.<ref name="cartoon" /> Some elements can form multiple simple substances with different structures: these are called [[allotrope]]s. For example, [[diamond]] and [[graphite]] are two allotropes of carbon.<ref name="Scerri14" />{{efn|The boundary between dispersion forces and metallic bonding is gradual, like that between ionic and covalent bonding. Characteristic metallic properties do not appear in small mercury clusters, but do appear in large ones.<ref>{{cite journal |last1=Pastor |first1=G. M. |last2=Stampfli |first2=P. |last3=Bennemann |first3=K. |date=1988 |title=On the transition from Van der Waals- to metallic bonding in Hg-clusters as a function of cluster size |url= |journal=Physica Scripta |volume=38 |issue=4 |pages=623–626 |doi=10.1088/0031-8949/38/4/022 |bibcode=1988PhyS...38..623P |s2cid=250842014 }}</ref>}}

The metallicity of an element can be predicted from electronic properties. When atomic orbitals overlap during metallic or covalent bonding, they create both bonding and antibonding [[molecular orbital]]s of equal capacity, with the antibonding orbitals of higher energy. Net bonding character occurs when there are more electrons in the bonding orbitals than there are in the antibonding orbitals. Metallic bonding is thus possible when the number of electrons delocalized by each atom is less than twice the number of orbitals contributing to the overlap. This is the situation for elements in groups 1 through 13; they also have too few valence electrons to form giant covalent structures where all atoms take equivalent positions, and so almost all of them metallise. The exceptions are hydrogen and boron, which have too high an ionisation energy. Hydrogen thus forms a covalent H<sub>2</sub> molecule, and boron forms a giant covalent structure based on icosahedral B<sub>12</sub> clusters. In a metal, the bonding and antibonding orbitals have overlapping energies, creating a single band that electrons can freely flow through, allowing for electrical conduction.<ref name=Siekierski>Siekierski and Burgess, pp. 60–66</ref>

[[File:Solid state electronic band structure.svg|thumb|upright=2.0|Graph of carbon atoms being brought together to form a diamond crystal, demonstrating formation of the electronic band structure and band gap. The right graph shows the energy levels as a function of the spacing between atoms. When far apart ''(right side of graph)'' all the atoms have discrete valence orbitals ''p'' and ''s'' with the same energies. However, when the atoms come closer ''(left side)'', their electron orbitals begin to spatially overlap. The orbitals [[Hybridization (chemistry)|hybridize]] into ''N'' molecular orbitals each with a different energy, where ''N'' is the number of atoms in the crystal. Since ''N'' is such a large number, adjacent orbitals are extremely close together in energy so the orbitals can be considered a continuous energy band. At the actual diamond crystal cell size (denoted by ''a''), two bands are formed, called the valence and conduction bands, separated by a 5.5&nbsp;[[electronvolt|eV]] band gap. (Here only the valence 2s and 2p electrons have been illustrated; the 1s orbitals do not significantly overlap, so the bands formed from them are much narrower.)]]
In group 14, both metallic and covalent bonding become possible. In a diamond crystal, covalent bonds between carbon atoms are strong, because they have a small atomic radius and thus the nucleus has more of a hold on the electrons. Therefore, the bonding orbitals that result are much lower in energy than the antibonding orbitals, and there is no overlap, so electrical conduction becomes impossible: carbon is a nonmetal. However, covalent bonding becomes weaker for larger atoms and the energy gap between the bonding and antibonding orbitals decreases. Therefore, silicon and germanium have smaller [[band gap]]s and are [[semiconductor]]s at ambient conditions: electrons can cross the gap when thermally excited. (Boron is also a semiconductor at ambient conditions.) The band gap disappears in tin, so that tin and lead become metals.<ref name=Siekierski/> As the temperature rises, all nonmetals develop some semiconducting properties, to a greater or lesser extent depending on the size of the band gap. Thus metals and nonmetals may be distinguished by the temperature dependence of their electrical conductivity: a metal's conductivity lowers as temperature rises (because thermal motion makes it more difficult for the electrons to flow freely), whereas a nonmetal's conductivity rises (as more electrons may be excited to cross the gap).<ref name=steudel/>

Elements in groups 15 through 17 have too many electrons to form giant covalent molecules that stretch in all three dimensions. For the lighter elements, the bonds in small diatomic molecules are so strong that a condensed phase is disfavoured: thus nitrogen (N<sub>2</sub>), oxygen (O<sub>2</sub>), white phosphorus and yellow arsenic (P<sub>4</sub> and As<sub>4</sub>), sulfur and red selenium (S<sub>8</sub> and Se<sub>8</sub>), and the stable halogens (F<sub>2</sub>, Cl<sub>2</sub>, Br<sub>2</sub>, and I<sub>2</sub>) readily form covalent molecules with few atoms. The heavier ones tend to form long chains (e.g. red phosphorus, grey selenium, tellurium) or layered structures (e.g. carbon as graphite, black phosphorus, grey arsenic, antimony, bismuth) that only extend in one or two rather than three dimensions. Both kinds of structures can be found as allotropes of phosphorus, arsenic, and selenium, although the long-chained allotropes are more stable in all three. As these structures do not use all their orbitals for bonding, they end up with bonding, nonbonding, and antibonding bands in order of increasing energy. Similarly to group 14, the band gaps shrink for the heavier elements and free movement of electrons between the chains or layers becomes possible. Thus for example black phosphorus, black arsenic, grey selenium, tellurium, and iodine are semiconductors; grey arsenic, antimony, and bismuth are [[semimetal]]s (exhibiting quasi-metallic conduction, with a very small band overlap); and polonium and probably astatine are true metals.<ref name=Siekierski/> Finally, the natural group 18 elements all stay as individual atoms.<ref name=Siekierski/>{{efn|All this describes the situation at standard pressure. Under sufficiently high pressure, the band gaps of any solid drop to zero and metallization occurs. Thus for example at about 170&nbsp;[[bar (unit)|kbar]] iodine becomes a metal,<ref name=Siekierski/> and [[metallic hydrogen]] should form at pressures of about four million atmospheres.<ref>{{cite journal |last1=McMinis |first1=J. |last2=Clay |first2=R.C. |last3=Lee |first3=D. |last4=Morales |first4=M.A. |year=2015 |title=Molecular to Atomic Phase Transition in Hydrogen under High Pressure |journal=[[Physical Review Letters|Phys. Rev. Lett.]] |volume=114 |issue=10 |page=105305 |doi=10.1103/PhysRevLett.114.105305 |pmid=25815944 |bibcode=2015PhRvL.114j5305M|doi-access=free }}</ref> See [[metallization pressure]] for values for all nonmetals.}}

The dividing line between metals and nonmetals is roughly diagonal from top left to bottom right, with the transition series appearing to the left of this diagonal (as they have many available orbitals for overlap). This is expected, as metallicity tends to be correlated with electropositivity and the willingness to lose electrons, which increases right to left and up to down. Thus the metals greatly outnumber the nonmetals. Elements near the borderline are difficult to classify: they tend to have properties that are intermediate between those of metals and nonmetals, and may have some properties characteristic of both. They are often termed semimetals or [[metalloid]]s.<ref name="cartoon" /> The term "semimetal" used in this sense should not be confused with its strict physical sense having to do with band structure: bismuth is physically a semimetal, but is generally considered a metal by chemists.<ref>{{cite journal |last1=Hawkes |first1=Stephen J. |date=2001 |title=Semimetallicity? |journal=Journal of Chemical Education |volume=78 |issue=12 |page=1686 |doi=10.1021/ed078p1686|bibcode=2001JChEd..78.1686H }}</ref>

The following table considers the most stable allotropes at standard conditions. The elements coloured yellow form simple substances that are well-characterised by metallic bonding. Elements coloured light blue form giant network covalent structures, whereas those coloured dark blue form small covalently bonded molecules that are held together by weaker [[van der Waals force]]s. The noble gases are coloured in violet: their molecules are single atoms and no covalent bonding occurs. Greyed-out cells are for elements which have not been prepared in sufficient quantities for their most stable allotropes to have been characterized in this way. Theoretical considerations and current experimental evidence suggest that all of those elements would metallise if they could form condensed phases,<ref name=Siekierski/> except perhaps for oganesson.<ref name="semiconductor">{{cite journal |last1=Mewes |first1=Jan-Michael |last2=Smits |first2=Odile Rosette |first3=Paul |last3=Jerabek |first4=Peter |last4=Schwerdtfeger |date=25 July 2019 |title=Oganesson is a Semiconductor: On the Relativistic Band-Gap Narrowing in the Heaviest Noble-Gas Solids |journal=Angewandte Chemie |volume=58 |issue=40 |pages=14260–14264|doi=10.1002/anie.201908327|pmid=31343819|pmc=6790653}}</ref>{{efn|Descriptions of the structures formed by the elements can be found throughout Greenwood and Earnshaw. There are two borderline cases. Arsenic's most stable form conducts electricity like a metal, but the bonding is significantly more localized to the nearest neighbours than it is for the similar structures of antimony and bismuth,<ref>{{cite book |last=Smith |first=J. D. |date=1973 |title=The Chemistry of Arsenic, Antimony and Bismuth |publisher=Pergamon Press |page=556 |isbn=}}</ref> and unlike normal metals it does not have a long liquid range, but rather sublimes instead. Hence its structure is better treated as network covalent.<ref>{{cite book |last1=Rayner-Canham |first1=Geoff |last2=Overton |first2=Tina |author-link= |date=2008 |title=Descriptive Inorganic Chemistry |edition=5th |url= |location=New York |publisher=W. H. Freeman and Company |page=194 |isbn=978-1-4292-2434-5}}</ref> Carbon as [[graphite]] shows metallic conduction parallel to its planes, but is a semiconductor perpendicular to them. Some computations predict copernicium and flerovium to be nonmetallic,<ref name=CRNL/><ref name=Florez/> but the most recent experiments on them suggest that they are metallic.<ref name=superheavy/><ref name=Ingo/><ref name=Yakushev/> Astatine is calculated to metallise at standard conditions,<ref name="Hermann">{{cite journal
|doi=10.1103/PhysRevLett.111.116404|title=Condensed Astatine: Monatomic and Metallic|year=2013|last1=Hermann|first1=A.|last2=Hoffmann|first2=R.|last3=Ashcroft|first3=N. W.|journal=Physical Review Letters|volume=111|issue=11|pages=116404-1–116404-5|bibcode=2013PhRvL.111k6404H|pmid=24074111}}</ref> so presumably tennessine should as well.<ref>{{cite news |last=Ball |first=Philip |date=13 September 2013 |title=
Metallic properties predicted for astatine |url=https://www.chemistryworld.com/news/metallic-properties-predicted-for-astatine/6582.article |work=Chemistry World |location= |access-date=7 April 2023}}</ref>}}

{{Periodic table (simple substance bonding)}}
<gallery mode="packed">
File:Iron electrolytic and 1cm3 cube.jpg|Iron, a metal
Sulfur - El Desierto mine, San Pablo de Napa, Daniel Campos Province, Potosí, Bolivia.jpg|Sulfur, a nonmetal
Arsen 1a.jpg|Arsenic, an element often called a semi-metal or metalloid
</gallery>

Generally, metals are shiny and dense.<ref name="cartoon" /> They usually have high melting and boiling points due to the strength of the metallic bond, and are often malleable and ductile (easily stretched and shaped) because the atoms can move relative to each other without breaking the metallic bond.<ref name="chemguidem">{{cite web |url=https://www.chemguide.co.uk/atoms/structures/metals.html |title=Metallic Structures |last=Clark |first=Jim |date=2012 |website=Chemguide |access-date=30 March 2021 |archive-date=24 April 2021 |archive-url=https://web.archive.org/web/20210424070514/https://www.chemguide.co.uk/atoms/structures/metals.html |url-status=live }}</ref> They conduct electricity because their electrons are free to move in all three dimensions. Similarly, they conduct heat, which is transferred by the electrons as extra [[kinetic energy]]: they move faster. These properties persist in the liquid state, as although the crystal structure is destroyed on melting, the atoms still touch and the metallic bond persists, though it is weakened.<ref name="chemguidem" /> Metals tend to be reactive towards nonmetals.<ref name="cartoon" /> Some exceptions can be found to these generalizations: for example, beryllium, chromium,<ref name=raynercanham/> manganese,<ref name="Holl">{{cite book|publisher=Walter de Gruyter|date=1985|edition=91–100 |pages=1110–1117|isbn=978-3-11-007511-3|title=Lehrbuch der Anorganischen Chemie|first=Arnold F.|last=Holleman|author2=Wiberg, Egon|author3=Wiberg, Nils|language=de|chapter=Mangan}}</ref> antimony,<ref name="wiberg_holleman">{{cite book|title=Inorganic chemistry|author=Wiberg, Egon|author2=Wiberg, Nils|author3=Holleman, Arnold Frederick|name-list-style=amp|publisher=Academic Press|date=2001|isbn=978-0-12-352651-9|page=758}}</ref> bismuth,<ref name="CRC">{{cite book| first = C. R.| last = Hammond| pages = [https://archive.org/details/crchandbookofche81lide/page/4 4–1<!-- not a range -->]| title = The Elements, in Handbook of Chemistry and Physics| edition = 81st| location = Boca Raton (FL, US)| publisher = CRC press| isbn = 978-0-8493-0485-9| date = 2004| url-access = registration| url = https://archive.org/details/crchandbookofche81lide/page/4}}</ref> and uranium are brittle (not an exhaustive list);<ref name=raynercanham/> chromium is extremely hard;<ref name=r1>{{cite book|editor=G.V. Samsonov|chapter=Mechanical Properties of the Elements|doi=10.1007/978-1-4684-6066-7_7|isbn=978-1-4684-6066-7|url=https://ihtik.lib.ru/2011.08_ihtik_nauka-tehnika/2011.08_ihtik_nauka-tehnika_3560.rar|publisher=IFI-Plenum|place=New York, USA|year=1968|pages=387–446|archive-url=https://web.archive.org/web/20150402123344/https://ihtik.lib.ru/2011.08_ihtik_nauka-tehnika/2011.08_ihtik_nauka-tehnika_3560.rar|archive-date=2 April 2015 |title=Handbook of the Physicochemical Properties of the Elements }}</ref> gallium, rubidium, caesium, and mercury are liquid at or close to room temperature;{{efn|See [[melting points of the elements (data page)]]. The same is probably true of francium, but due to its extreme instability, this has never been experimentally confirmed. Copernicium and flerovium are expected to be liquids,<ref name="CRNL">{{cite journal |last1=Mewes |first1=J.-M. |last2=Smits |first2=O. R. |last3=Kresse |first3=G. |last4=Schwerdtfeger |first4=P. |title=Copernicium is a Relativistic Noble Liquid |journal=Angewandte Chemie International Edition |date=2019 |volume=58|issue=50|pages=17964–17968|doi=10.1002/anie.201906966 |pmid=31596013 |pmc=6916354 |url=}}</ref><ref name=Florez>{{cite journal |last1=Florez |first1=Edison |last2=Smits |first2=Odile R. |last3=Mewes |first3=Jan-Michael |last4=Jerabek |first4=Paul |last5=Schwerdtfeger |first5=Peter |date=2022 |title=From the gas phase to the solid state: The chemical bonding in the superheavy element flerovium |journal=The Journal of Chemical Physics |volume=157 |issue=6 |page=064304 |doi=10.1063/5.0097642|pmid=35963734 |bibcode=2022JChPh.157f4304F |s2cid=250539378 }}</ref> similar to mercury, and experimental evidence suggests that they are metals.<ref name="superheavy">
{{Cite web
|last1=Gäggeler
|first1=H. W.
|year=2007
|title=Gas Phase Chemistry of Superheavy Elements
|url=https://lch.web.psi.ch/files/lectures/TexasA&M/TexasA&M.pdf
|pages=26–28
|publisher=[[Paul Scherrer Institute]]
|archive-url=https://web.archive.org/web/20120220090755/https://lch.web.psi.ch/files/lectures/TexasA%26M/TexasA%26M.pdf
|archive-date=20 February 2012
}}</ref><ref name=Ingo>{{cite news |last=Ingo |first=Peter |date=15 September 2022 |title=Study shows flerovium is the most volatile metal in the periodic table |url=https://phys.org/news/2022-09-flerovium-volatile-metal-periodic-table.html |work=phys.org<!--but provided by GSI Helmholtz--> |location= |access-date=22 November 2022}}</ref><ref name=Yakushev>{{cite journal |last1=Yakushev |first1=A. |last2=Lens |first2=L. |first3=Ch. E. |last3=Düllmann |first4=J. |last4=Khuyagbaatar |first5=E. |last5=Jäger |first6=J. |last6=Krier |first7=J. |last7=Runke |first8=H. M. |last8=Albers |first9=M. |last9=Asai |first10=M. |last10=Block |first11=J. |last11=Despotopulos |first12=A. |last12=Di Nitto |first13=K. |last13=Eberhardt |first14=U. |last14=Forsberg |first15=P. |last15=Golubev |first16=M. |last16=Götz |first17=S. |last17=Götz |first18=H. |last18=Haba |first19=L. |last19=Harkness-Brennan |first20=R.-D. |last20=Herzberg |first21=F. P. |last21=Heßberger |first22=D. |last22=Hinde |first23=A. |last23=Hübner |first24=D. |last24=Judson |first25=B. |last25=Kindler |first26=Y. |last26=Komori |first27=J. |last27=Konki |first28=J. V. |last28=Kratz |first29=N. |last29=Kurz |first30=M. |last30=Laatiaoui |first31=S. |last31=Lahiri |first32=B. |last32=Lommel |first33=M. |last33=Maiti |first34=A. K. |last34=Mistry |first35=Ch. |last35=Mokry |first36=K. J. |last36=Moody |first37=Y. |last37=Nagame |first38=J. P. |last38=Omtvedt |first39=P. |last39=Papadakis |first40=V. |last40=Pershina |first41=D. |last41=Rudolph |first42=L. G. |last42=Samiento |first43=T. K. |last43=Sato |first44=M. |last44=Schädel |first45=P. |last45=Scharrer |first46=B. |last46=Schausten |first47=D. A. |last47=Shaughnessy |first48=J. |last48=Steiner |first49=P. |last49=Thörle-Pospiech |first50=A. |last50=Toyoshima |first51=N. |last51=Trautmann |first52=K. |last52=Tsukada |first53=J. |last53=Uusitalo |first54=K.-O. |last54=Voss |first55=A. |last55=Ward |first56=M. |last56=Wegrzecki |first57=N. |last57=Wiehl |first58=E. |last58=Williams |first59=V. |last59=Yakusheva |display-authors=3 |date=25 August 2022 |title=On the adsorption and reactivity of element 114, flerovium |journal=Frontiers in Chemistry |volume=10 |issue=976635 |page=976635 |doi=10.3389/fchem.2022.976635 |pmid=36092655 |pmc=9453156 |bibcode=2022FrCh...10.6635Y |doi-access=free }}</ref>}} and [[noble metal]]s such as gold are chemically very inert.<ref>{{cite journal |doi=10.1038/376238a0 |title=Why gold is the noblest of all the metals |date=1995 |last1=Hammer |first1=B. |last2=Norskov |first2=J. K. |journal=Nature |volume=376 |issue=6537 |pages=238–240 |bibcode=1995Natur.376..238H|s2cid=4334587 }}</ref><ref>{{cite journal |doi=10.1103/PhysRevB.6.4370 |title=Optical Constants of the Noble Metals |date=1972 |last1=Johnson |first1=P. B. |last2=Christy |first2=R. W. |journal=Physical Review B |volume=6 |issue=12 |pages=4370–4379 |bibcode=1972PhRvB...6.4370J}}</ref>

Nonmetals exhibit different properties. Those forming giant covalent crystals exhibit high melting and boiling points, as it takes considerable energy to overcome the strong covalent bonds. Those forming discrete molecules are held together mostly by dispersion forces, which are more easily overcome; thus they tend to have lower melting and boiling points,<ref>{{cite web |url=https://www.chemguide.co.uk/inorganic/period3/elementsphys.html |title=Atomic and Physical Properties of the Period 3 Elements |last=Clark |first=Jim |date=2018 |website=Chemguide |access-date=30 March 2021 |archive-date=22 April 2021 |archive-url=https://web.archive.org/web/20210422142013/https://www.chemguide.co.uk/inorganic/period3/elementsphys.html |url-status=live }}</ref> and many are liquids or gases at room temperature.<ref name="cartoon" /> Nonmetals are often dull-looking. They tend to be reactive towards metals, except for the noble gases, which are inert towards most substances.<ref name="cartoon" /> They are brittle when solid as their atoms are held tightly in place. They are less dense and conduct electricity poorly,<ref name="cartoon" /> because there are no mobile electrons.<ref name="group4">{{cite web |url=https://www.chemguide.co.uk/inorganic/group4/properties.html |title=The Trend From Non-Metal to Metal In the Group 4 Elements |last=Clark |first=Jim |date=2015 |website=Chemguide |access-date=30 March 2021 |archive-date=27 April 2021 |archive-url=https://web.archive.org/web/20210427234147/https://www.chemguide.co.uk/inorganic/group4/properties.html |url-status=live }}</ref> Near the borderline, band gaps are small and thus many elements in that region are semiconductors, such as silicon, germanium,<ref name="group4" /> and tellurium.<ref name=Siekierski/> Selenium has both a semiconducting grey allotrope and an insulating red allotrope; arsenic has a metallic grey allotrope, a semiconducting black allotrope, and an insulating yellow allotrope (though the last is unstable at ambient conditions).<ref name=steudel/> Again there are exceptions; for example, diamond has the highest thermal conductivity of all known materials, greater than any metal.<ref name=PNU>{{cite journal |doi=10.1103/PhysRevLett.70.3764 |title=Thermal conductivity of isotopically modified single crystal diamond |year=1993 |last1=Wei |first1=Lanhua |last2=Kuo |first2=P. K. |last3=Thomas |first3=R. L. |last4=Anthony |first4=T. R. |last5=Banholzer |first5=W. F. |journal=Physical Review Letters |volume=70 |issue=24 |pages=3764–3767 |pmid=10053956 |bibcode=1993PhRvL..70.3764W}}</ref>

It is common to designate a class of metalloids straddling the boundary between metals and nonmetals, as elements in that region are intermediate in both physical and chemical properties.<ref name="cartoon" /> However, no consensus exists in the literature for precisely which elements should be so designated. When such a category is used, silicon, germanium, arsenic, and tellurium are almost always included, and boron and antimony usually are; but most sources include other elements as well, without agreement on which extra elements should be added, and some others subtract from this list instead.{{efn|See [[lists of metalloids]]. For example, a periodic table used by the American Chemical Society includes polonium as a metalloid,<ref name="ACS" /> but one used by the Royal Society of Chemistry does not,<ref>{{cite web |url=https://www.rsc.org/periodic-table |title=Periodic Table |date=2021 |website=www.rsc.org |publisher=[[Royal Society of Chemistry]] |access-date=27 March 2021 |archive-date=21 March 2021 |archive-url=https://web.archive.org/web/20210321033913/https://www.rsc.org/periodic-table |url-status=live }}</ref> and that included in the ''[[Encyclopædia Britannica]]'' does not refer to metalloids or semi-metals at all.<ref name="EB" /> Classification can change even within a single work. For example, Sherwin and Weston's ''Chemistry of the Non-Metallic Elements'' (1966) has a periodic table on p. 7 classifying antimony as a nonmetal, but on p. 115 it is called a metal.<ref>{{cite book |last1=Sherwin |first1=E. |last2=Weston |first2=G. J. |editor=Spice, J. E. |date=1966 |title=Chemistry of the Non-Metallic Elements |publisher=Pergamon Press |isbn=978-1-4831-3905-0}}</ref>|name=metalloids}} For example, unlike all the other elements generally considered metalloids or nonmetals, antimony's only stable form has metallic conductivity. Moreover, the element resembles bismuth and, more generally, the other p-block metals in its physical and chemical behaviour. On this basis some authors have argued that it is better classified as a metal than as a metalloid.<ref name=raynercanham/><ref name=hawkes>{{cite journal |last1=Hawkes |first1=Stephen J. |date=2001 |title=Semimetallicity? |url= |journal=Journal of Chemical Education |volume=78 |issue=12 |pages=1686–1687 |doi=10.1021/ed078p1686 |bibcode=2001JChEd..78.1686H |access-date=}}</ref><ref name=steudel>{{cite book |last1=Steudel |first1=Ralf |first2=David |last2=Scheschkewitz |author-link= |date=2020 |title=Chemistry of the Non-Metals |url= |location= |publisher=Walter de Gruyter |pages=154–155, 425, 436 |isbn=978-3-11-057805-8 |quote=In Group 15 of the Periodic Table, as in both neighboring groups, the metallic character increases when going down. More specifically, there is a transition from a purely non-metallic element (N) via elements with nonmetallic and metallic modifications to purely metallic elements (Sb, Bi). This chapter addresses the two elements besides nitrogen, which are clearly nonmetallic under standard conditions: phosphorus and arsenic. The chemistry of arsenic, however, is only briefly described as many of the arsenic compounds resemble the corresponding phosphorus species.}}</ref> On the other hand, selenium has some semiconducting properties in its most stable form (though it also has insulating allotropes) and it has been argued that it should be considered a metalloid<ref name=hawkes/> – though this situation also holds for phosphorus,<ref name=steudel/> which is a much rarer inclusion among the metalloids.{{efn|name=metalloids}}