Editing Metalloid (section)

==Elements less commonly recognised as metalloids==

===Carbon===
{{Main|Carbon}}
[[File:Graphite2.jpg|thumb|right|[[Carbon]] (as [[graphite]]). [[Delocalized electron|Delocalized valence electrons]] within the layers of graphite give it a metallic appearance.<ref>[[#Hill2000|Hill & Holman 2000, p.&nbsp;124]]</ref>|alt=A shiny grey-black cuboid nugget with a rough surface.]]
Carbon is ordinarily classified as a nonmetal<ref>[[#Chang2002|Chang 2002, p.&nbsp;314]]</ref> but has some metallic properties and is occasionally classified as a metalloid.<ref>[[#Kent1950|Kent 1950, pp.&nbsp;1–2]]; [[#Clark1960|Clark 1960, p.&nbsp;588]]; [[#Warren1981|Warren & Geballe 1981]]</ref> [[Graphite|Hexagonal graphitic carbon]] (graphite) is the most thermodynamically stable [[allotrope]] of carbon under ambient conditions.<ref>[[#Housecroft2008|Housecroft & Sharpe 2008, p.&nbsp;384]]; [[#IUPAC2006|IUPAC 2006–, rhombohedral graphite entry]]</ref> It has a lustrous appearance<ref>[[#Mingos1998|Mingos 1998, p.&nbsp;171]]</ref> and is a fairly good electrical conductor.<ref>[[#Wiberg2001|Wiberg 2001, p.&nbsp;781]]</ref> Graphite has a layered structure. Each layer consists of carbon atoms bonded to three other carbon atoms in a [[hexagonal lattice]] arrangement. The layers are stacked together and held loosely by [[van der Waals force]]s and [[delocalized electron|delocalized valence electrons]].<ref>[[#Charlier|Charlier, Gonze & Michenaud 1994]]</ref>

The electrical conductivity of graphite is high parallel to its planes (30 kS/cm at 25°C), and decreases with increasing temperature, indicating [[Semimetal|semimetallic]] behaviour along that direction. Perpendicular to the planes, graphite behaves as a [[semiconductor]]: the conductivity is low (5 S/cm) but increases as the temperature rises.<ref name="Atkins320">[[Metalloid#Atkins2006|Atkins et al. 2006, pp.&nbsp;320–21]]</ref>{{refn|1=Liquid carbon may<ref>[[#Savvatimskiy2005|Savvatimskiy 2005, p.&nbsp;1138]]</ref> or may not<ref>[[#Togaya2000|Togaya 2000]]</ref> be a metallic conductor, depending on pressure and temperature; see also.<ref>[[#Savvatimskiy2009|Savvatimskiy 2009]]</ref>|group=n}} The allotropes of carbon, including graphite, can accept foreign atoms or compounds into their structures via substitution, [[intercalation (chemistry)|intercalation]], or [[dopant|doping]]. The resulting materials are sometimes referred to as "carbon alloys".<ref>[[#Inagaki2000|Inagaki 2000, p.&nbsp;216]]; [[#Yasuda2003|Yasuda et al. 2003, pp.&nbsp;3–11]]</ref> Carbon can form ionic salts, including a hydrogen sulfate, perchlorate, and nitrate (C{{su|b=24|p=+}}X<sup>−</sup>.2HX, where X = HSO<sub>4</sub>, ClO<sub>4</sub>; and C{{su|b=24|p=+}}NO{{su|b=3|p=–}}.3HNO<sub>3</sub>).<ref>[[#O'Hare|O'Hare 1997, p.&nbsp;230]]</ref>{{refn|1=For the sulfate, the method of preparation is (careful) direct oxidation of graphite in concentrated sulfuric acid by an [[oxidising agent]], such as [[nitric acid]], [[chromium trioxide]] or [[ammonium persulfate]]; in this instance the concentrated sulfuric acid is acting as an [[inorganic nonaqueous solvent]].|group=n}} In [[organic chemistry]], carbon can form complex cations{{snd}}termed [[carbocation|''carbocations'']]{{snd}}in which the positive charge is on the carbon atom; examples are [[carbenium ion|{{chem|CH|3|+}}]] and [[carbonium ion|{{chem|CH|5|+}}]], and their derivatives.<ref>[[#Traynham1989|Traynham 1989, pp.&nbsp;930–31]]; [[#Prakash1997|Prakash & Schleyer 1997]]</ref>

Graphite is an established solid lubricant and behaves as a semiconductor in a direction perpendicular to its planes.<ref name=Atkins320/> Most of its chemistry is nonmetallic;<ref>[[#Bailar1989|Bailar et al. 1989, p.&nbsp;743]]</ref> it has a relatively high ionization energy<ref>[[#Moore1985|Moore et al. 1985]]</ref> and, compared to most metals, a relatively high electronegativity.<ref>[[#House2010|House & House 2010, p.&nbsp;526]]</ref> Carbon can form anions such as C<sup>4−</sup> ([[methanide]]), C{{su|b=2|p=2–}} ([[acetylide]]), and C{{su|b=4|p=3–}} ([[Sesquicarbide|sesquicarbide or allylenide]]), in compounds with metals of main groups 1–3, and with the [[lanthanide]]s and [[actinide]]s.<ref>[[#Wiberg2001|Wiberg 2001, p.&nbsp;798]]</ref> Its oxide [[carbon dioxide|CO<sub>2</sub>]] forms [[carbonic acid]] H<sub>2</sub>CO<sub>3</sub>.<ref>[[#Eagleson1994|Eagleson 1994, p.&nbsp;175]]</ref>{{refn|1=Only a small fraction of dissolved CO<sub>2</sub> is present in water as carbonic acid so, even though H<sub>2</sub>CO<sub>3</sub> is a medium-strong acid, solutions of carbonic acid are only weakly acidic.<ref>[[#Atkins2006|Atkins et al. 2006, p.&nbsp;121]]</ref>|group=n}}

===Aluminium===
{{Main|Aluminium}}
[[File:Aluminium-4.jpg|thumb|left|High purity [[aluminium]] is much softer than its familiar [[aluminium alloys|alloys]]. People who handle it for the first time often ask if it is the real thing.<ref>[[#Russell2005|Russell & Lee 2005, pp.&nbsp;358–59]]</ref>|alt=A silvery white steam-iron shaped lump with semi-circular striations along the width of its top surface and rough furrows in the middle portion of its left edge.]]
Aluminium is ordinarily classified as a metal.<ref>[[#Keevil|Keevil 1989, p.&nbsp;103]]</ref> It is lustrous, malleable and ductile, and has high electrical and thermal conductivity. Like most metals it has a [[close-packed]] crystalline structure,<ref>[[#Russell2005|Russell & Lee 2005, pp.&nbsp;358–60 et seq]]</ref>  and forms a cation in aqueous solution.<ref>[[#Harding|Harding, Janes & Johnson 2002, p.&nbsp;118]]</ref>

It has some properties that are unusual for a metal; taken together,<ref name="Metcalfe et al. 1974, p.539">[[#Metcalfe1974|Metcalfe, Williams & Castka 1974, p.&nbsp;539]]</ref> these are sometimes used as a basis to classify aluminium as a metalloid.<ref>[[#Cobb2005|Cobb & Fetterolf 2005, p.&nbsp;64]]; [[#Metcalfe1974|Metcalfe, Williams & Castka 1974, p.&nbsp;539]]</ref> Its crystalline structure shows some evidence of [[Bonding in solids#Properties|directional bonding]].<ref>[[#Ogata2002|Ogata, Li & Yip 2002]]; [[#Boyer2004|Boyer et al. 2004, p.&nbsp;1023]]; [[#Russell2005|Russell & Lee 2005, p.&nbsp;359]]</ref> Aluminium bonds covalently in most compounds.<ref>[[#Cooper1968|Cooper 1968, p.&nbsp;25]]; [[#Henderson2000|Henderson 2000, p.&nbsp;5]]; [[#Silberberg2006|Silberberg 2006, p.&nbsp;314]]</ref> The oxide [[aluminium oxide|Al<sub>2</sub>O<sub>3</sub>]] is amphoteric<ref>[[#Wiberg2001|Wiberg 2001, p.&nbsp;1014]]</ref> and a conditional glass-former.<ref name=Rao22/> Aluminium can form anionic [[aluminate]]s,<ref name="Metcalfe et al. 1974, p.539"/> such behaviour being considered nonmetallic in character.<ref name="Hamm 1969, p.653">[[#Hamm1969|Hamm 1969, p.&nbsp;653]]</ref>

Classifying aluminium as a metalloid has been disputed<ref>[[#Daub1996|Daub & Seese 1996, pp.&nbsp;70, 109]]: "Aluminum is not a metalloid but a metal because it has mostly metallic properties."; [[#Denniston2004|Denniston, Topping & Caret 2004, p.&nbsp;57]]: "Note that aluminum (Al) is classified as a metal, not a metalloid."; [[#Hasan2009|Hasan 2009, p.&nbsp;16]]: "Aluminum does not have the characteristics of a metalloid but rather those of a metal."</ref> given its many metallic properties. It is therefore, arguably, an exception to the mnemonic that elements adjacent to the metal–nonmetal dividing line are metalloids.<ref>[[#Holt2007|Holt, Rinehart & Wilson c. 2007]]</ref>{{refn|1=A mnemonic that captures the elements commonly recognised as metalloids goes: ''Up, up-down, up-down, up&nbsp;... are the metalloids!''<ref>[[#Tuthill2011|Tuthill 2011]]</ref>|group=n}}

Stott<ref>[[#Stott1956|Stott 1956, p.&nbsp;100]]</ref> labels aluminium as a weak metal. It has the physical properties of a metal but some of the chemical properties of a nonmetal. Steele<ref>[[#Steele1966|Steele 1966, p.&nbsp;60]]</ref> notes the paradoxical chemical behaviour of aluminium: "It resembles a weak metal in its amphoteric oxide and in the covalent character of many of its compounds&nbsp;... Yet it is a highly [[electronegativity#Electropositivity|electropositive]] metal&nbsp;... [with] a [[table of standard electrode potentials|high negative]] electrode potential". Moody<ref>[[#Moody|Moody 1991, p.&nbsp;303]]</ref> says that, "aluminium is on the 'diagonal borderland' between metals and non-metals in the chemical sense."

===Selenium===
{{Main|Selenium}}
[[File:Selenium black (cropped).jpg|thumb|right|upright|Grey [[selenium]], being a [[photoconductor]], conducts electricity around 1,000 times better when light falls on it, a property used since the mid-1870s in various light-sensing applications<ref>[[#Emsley2001|Emsley 2001, p.&nbsp;382]]</ref>|alt=A small glass jar filled with small dull grey concave buttons. The pieces of selenium look like tiny mushrooms without their stems.]]
Selenium shows borderline metalloid or nonmetal behaviour.<ref>[[#Young2010|Young et al. 2010, p.&nbsp;9]]; [[#Craig2003|Craig & Maher 2003, p.&nbsp;391]]. Selenium is "near metalloidal".</ref>{{refn|1=[[Eugene G. Rochow|Rochow]],<ref>[[#Rochow1957|Rochow 1957]]</ref> who later wrote his 1966 monograph ''The metalloids'',<ref>[[#Rochow1966|Rochow 1966, p.&nbsp;224]]</ref> commented that, "In some respects selenium acts like a metalloid and tellurium certainly does".|group=n}}

Its most stable form, the grey [[trigonal crystal system|trigonal]] allotrope, is sometimes called "metallic" selenium because its electrical conductivity is several orders of magnitude greater than that of the red [[monoclinic crystal system|monoclinic]] form.<ref>[[#Moss1952|Moss 1952, p.&nbsp;192]]</ref> The metallic character of selenium is further shown by its lustre,<ref name="Glinka 1965, p.356">[[#Glinka1965|Glinka 1965, p.&nbsp;356]]</ref> and its crystalline structure, which is thought to include weakly "metallic" interchain bonding.<ref>[[#Evans1966|Evans 1966, pp.&nbsp;124–25]]</ref> Selenium can be drawn into thin threads when molten and viscous.<ref>[[#Regnault1853|Regnault 1853, p.&nbsp;208]]</ref> It shows reluctance to acquire "the high positive oxidation numbers characteristic of nonmetals".<ref>[[#Scott1962|Scott & Kanda 1962, p.&nbsp;311]]</ref> It can form cyclic polycations (such as Se{{su|b=8|p=2+}}) when dissolved in [[oleum]]s<ref>[[#Cotton1999|Cotton et al. 1999, pp.&nbsp;496, 503–04]]</ref> (an attribute it shares with sulfur and tellurium), and a hydrolysed cationic salt in the form of trihydroxoselenium(IV) perchlorate <span style="white-space: nowrap">[Se(OH)<sub>3</sub>]<sup>+</sup>·ClO{{su|b=4|p=–}}.<ref>[[#Arlman1939|Arlman 1939]]; [[#Bagnall1966|Bagnall 1966, pp.&nbsp;135, 142–43]]</ref></span>

The nonmetallic character of selenium is shown by its brittleness<ref name="Glinka 1965, p.356"/> and the low electrical conductivity (~10<sup>−9</sup> to 10<sup>−12</sup>&nbsp;S•cm<sup>−1</sup>) of its highly purified form.<ref name="Kozyrev">[[#Kozyrev1959|Kozyrev 1959, p.&nbsp;104]]; [[#Chizhikov1968|Chizhikov & Shchastlivyi 1968, p.&nbsp;25]]; [[#Glazov1969|Glazov, Chizhevskaya & Glagoleva 1969, p.&nbsp;86]]</ref> This is comparable to or less than that of [[bromine]] (7.95{{e|–12}}&nbsp;S•cm<sup>−1</sup>),<ref>[[#Chao1964|Chao & Stenger 1964]]</ref> a nonmetal. Selenium has the electronic band structure of a [[semiconductor]]<ref name="Berger 1997, pp.86–7">[[#Berger1997|Berger 1997, pp.&nbsp;86–87]]</ref> and retains its semiconducting properties in liquid form.<ref name="Berger 1997, pp.86–7"/> It has a relatively high<ref>[[#Snyder1966|Snyder 1966, p.&nbsp;242]]</ref> electronegativity (2.55 revised Pauling scale). Its reaction chemistry is mainly that of its nonmetallic anionic forms Se<sup>2−</sup>, SeO{{su|b=3|p=2−}} and SeO{{su|b=4|p=2−}}.<ref>[[#Fritz2008|Fritz & Gjerde 2008, p.&nbsp;235]]</ref>

Selenium is commonly described as a metalloid in the [[environmental chemistry]] literature.<ref>[[#Meyer2005|Meyer et al. 2005, p.&nbsp;284]]; [[#Manahan|Manahan 2001, p.&nbsp;911]]; [[#Szpunar|Szpunar et al. 2004, p.&nbsp;17]]</ref> It moves through the aquatic environment similarly to arsenic and antimony;<ref>[[#USEPA1988|US Environmental Protection Agency 1988, p.&nbsp;1]]; [[#Uden|Uden 2005, pp.&nbsp;347‒48]]</ref> its water-soluble salts, in higher concentrations, have a similar [[toxicology|toxicological profile]] to that of arsenic.<ref>[[#DeZuane|De Zuane 1997, p.&nbsp;93]]; [[#Dev|Dev 2008, pp.&nbsp;2‒3]]</ref>

===Polonium===
{{Main|Polonium}}
Polonium is "distinctly metallic" in some ways.<ref name="Cotton FA 1999, p.502">[[#Cotton1999|Cotton et al. 1999, p.&nbsp;502]]</ref> Both of its allotropic forms are metallic conductors.<ref name="Cotton FA 1999, p.502"/> It is soluble in acids, forming the rose-coloured Po<sup>2+</sup> cation and displacing hydrogen: Po + 2 H<sup>+</sup> → Po<sup>2+</sup> + H<sub>2</sub>.<ref>[[#Wiberg2001|Wiberg 2001, p.&nbsp;594]]</ref> Many polonium salts are known.<ref>[[#Greenwood2002|Greenwood & Earnshaw 2002, p.&nbsp;786]]; [[#Schwietzer2010|Schwietzer & Pesterfield 2010, pp.&nbsp;242–43]]</ref> The oxide [[polonium dioxide|PoO<sub>2</sub>]] is predominantly basic in nature.<ref name=Bagnall1966p41>[[#Bagnall1966|Bagnall 1966, p.&nbsp;41]]; [[#Nickless1968|Nickless 1968, p.&nbsp;79]]</ref> Polonium is a reluctant oxidizing agent, unlike its lightest congener oxygen: highly [[reducing agent|reducing conditions]] are required for the formation of the Po<sup>2−</sup> anion in aqueous solution.<ref>[[#Bagnall1990|Bagnall 1990, pp.&nbsp;313–14]]; [[#Lehto2011|Lehto & Hou 2011, p.&nbsp;220]]; [[#Siekierski2002|Siekierski & Burgess 2002, p.&nbsp;117]]: "The tendency to form X<sup>2−</sup> anions decreases down the Group [16 elements]&nbsp;..."</ref>

Whether polonium is ductile or brittle is unclear. It is predicted to be ductile based on its calculated [[Young's modulus#Relation among elastic constants|elastic constants]].<ref>[[#Legit|Legit, Friák & Šob 2010, pp.&nbsp;214118–18]]</ref> It has a simple [[cubic crystal system|cubic crystalline structure]]. Such a structure has few [[Slip (materials science)#slip systems|slip systems]] and "leads to very low ductility and hence low fracture resistance".<ref>[[#Halford|Manson & Halford 2006, pp.&nbsp;378, 410]]</ref>

Polonium shows nonmetallic character in its halides, and by the existence of [[polonide]]s. The halides have properties generally characteristic of nonmetal halides (being volatile, easily hydrolyzed, and soluble in [[organic solvent]]s).<ref>[[#Bagnall1957|Bagnall 1957, p.&nbsp;62]]; [[#Fernelius1982|Fernelius 1982, p.&nbsp;741]]</ref> Many metal polonides, obtained by heating the elements together at 500–1,000&nbsp;°C, and containing the Po<sup>2−</sup> anion, are also known.<ref>[[#Bagnall1966|Bagnall 1966, p.&nbsp;41]]; [[#Barrett2003|Barrett 2003, p.&nbsp;119]]</ref>

===Astatine===
{{Main|Astatine}}
As a [[halogen]], astatine tends to be classified as a nonmetal.<ref>[[#Hawkes2010|Hawkes 2010]]; [[#Holt2007|Holt, Rinehart & Wilson c. 2007]]; [[#Hawkes1999|Hawkes 1999, p.&nbsp;14]]; [[#Roza2009|Roza 2009, p.&nbsp;12]]</ref> It has some marked metallic properties<ref>[[#Keller1985|Keller 1985]]</ref> and is sometimes instead classified as either a metalloid<ref>[[#Harding2002|Harding, Johnson & Janes 2002, p.&nbsp;61]]</ref> or (less often) as a metal.{{refn|1=A further option is to include astatine both as a nonmetal and as a metalloid.<ref>[[#Long1986|Long & Hentz 1986, p.&nbsp;58]]</ref>|group=n}} Immediately following its production in 1940, early investigators considered it a metal.<ref>[[#Vasáros1985|Vasáros & Berei 1985, p.&nbsp;109]]</ref> In 1949 it was called the most noble (difficult to [[redox|reduce]]) nonmetal as well as being a relatively noble (difficult to oxidize) metal.<ref>[[#Haissinsky1949|Haissinsky & Coche 1949, p.&nbsp;400]]</ref> In 1950 astatine was described as a halogen and (therefore) a [[reactivity (chemistry)|reactive]] nonmetal.<ref>[[#Brownlee1950|Brownlee et al. 1950, p.&nbsp;173]]</ref> In 2013, on the basis of [[relativistic quantum chemistry|relativistic]] modelling, astatine was predicted to be a monatomic metal, with a [[Face-centred cubic|face-centred cubic crystalline structure]].<ref>[[#Hermann|Hermann, Hoffmann & Ashcroft 2013]]</ref>

Several authors have commented on the metallic nature of some of the properties of astatine. Since iodine is a semiconductor in the direction of its planes, and since the halogens become more metallic with increasing atomic number, it has been presumed that astatine would be a metal if it could form a condensed phase.<ref>[[#Siekierski2002|Siekierski & Burgess 2002, pp.&nbsp;65, 122]]</ref>{{refn|1=A visible piece of astatine would be immediately and completely vaporized because of the heat generated by its intense radioactivity.<ref>[[#Emsley2001|Emsley 2001, p.&nbsp;48]]</ref>|group=n}} Astatine may be metallic in the liquid state on the basis that elements with an [[enthalpy of vaporization]] (∆H<sub>vap</sub>) greater than ~42 kJ/mol are metallic when liquid.<ref name="Rao & Ganguly 1986">[[#Rao1986|Rao & Ganguly 1986]]</ref> Such elements include boron,{{refn|1=The literature is contradictory as to whether boron exhibits metallic conductivity in liquid form. Krishnan et al.<ref>[[#Krishnan1998|Krishnan et al. 1998]]</ref> found that liquid boron behaved like a metal. Glorieux et al.<ref>[[#Glorieux2001|Glorieux, Saboungi & Enderby 2001]]</ref> characterised liquid boron as a semiconductor, on the basis of its low electrical conductivity. Millot et al.<ref>[[#Millot2002|Millot et al. 2002]]</ref> reported that the emissivity of liquid boron was not consistent with that of a liquid metal.|group=n}} silicon, germanium, antimony, selenium, and tellurium. Estimated values for ∆H<sub>vap</sub> of [[Diatomic molecule|diatomic]] astatine are 50 kJ/mol or higher;<ref>[[#Vasáros1985|Vasáros & Berei 1985, p.&nbsp;117]]</ref> diatomic iodine, with a ∆H<sub>vap</sub> of 41.71,<ref>[[#Kaye1973|Kaye & Laby 1973, p.&nbsp;228]]</ref> falls just short of the threshold figure.

"Like typical metals, it [astatine] is precipitated by [[hydrogen sulfide]] even from strongly acid solutions and is displaced in a free form from sulfate solutions; it is deposited on the [[cathode]] on [[electrolysis]]."<ref>[[#Samsonov1968|Samsonov 1968, p.&nbsp;590]]</ref>{{refn|1=Korenman<ref>[[#Korenman1959|Korenman 1959, p.&nbsp;1368]]</ref> similarly noted that "the ability to precipitate with hydrogen sulfide distinguishes astatine from other halogens and brings it closer to bismuth and other [[heavy metal (chemistry)|heavy metals]]".|group=n}} Further indications of a tendency for astatine to behave like a [[heavy metal (chemistry)|(heavy) metal]] are: "...&nbsp;the formation of [[pseudohalide]] compounds&nbsp;... complexes of astatine cations&nbsp;... complex anions of trivalent astatine&nbsp;... as well as complexes with a variety of organic solvents".<ref>[[#Rossler1985|Rossler 1985, pp.&nbsp;143–44]]</ref> It has also been argued that astatine demonstrates cationic behaviour, by way of stable At<sup>+</sup> and AtO<sup>+</sup> forms, in strongly acidic aqueous solutions.<ref>[[#Champion2010|Champion et al. 2010]]</ref>

Some of astatine's reported properties are nonmetallic. It has been extrapolated to have the narrow liquid range ordinarily associated with nonmetals (mp 302&nbsp;°C; bp 337&nbsp;°C),<ref>[[#Borst1982|Borst 1982, pp.&nbsp;465, 473]]</ref> although experimental indications suggest a lower boiling point of about 230±3&nbsp;°C. Batsanov gives a calculated band gap energy for astatine of 0.7&nbsp;eV;<ref>[[#Batsanov1971|Batsanov 1971, p.&nbsp;811]]</ref> this is consistent with nonmetals (in physics) having separated [[valence band|valence]] and [[conduction band]]s and thereby being either semiconductors or insulators.<ref>[[#Swalin1962|Swalin 1962, p.&nbsp;216]]; [[#Feng2005|Feng & Lin 2005, p.&nbsp;157]]</ref> The chemistry of astatine in aqueous solution is mainly characterised by the formation of various anionic species.<ref>[[#Schwietzer2010|Schwietzer & Pesterfield 2010, pp.&nbsp;258–60]]</ref> Most of its known compounds resemble those of iodine,<ref>[[#Hawkes1999|Hawkes 1999, p.&nbsp;14]]</ref> which is a halogen and a nonmetal.<ref>[[#Olmsted1997|Olmsted & Williams 1997, p.&nbsp;328]]; [[#Daintith2004|Daintith 2004, p.&nbsp;277]]</ref> Such compounds include astatides (XAt), astatates (XAtO<sub>3</sub>), and [[monovalent ion|monovalent]] [[interhalogen compound]]s.<ref>[[#Eberle1985|Eberle1985, pp.&nbsp;213–16, 222–27]]</ref>

Restrepo et al.<ref>[[#Restrepo2004|Restrepo et al. 2004, p.&nbsp;69]]; [[#Restrepo2006|Restrepo et al. 2006, p.&nbsp;411]]</ref> reported that astatine appeared to be more polonium-like than halogen-like. They did so on the basis of detailed comparative studies of the known and interpolated properties of 72 elements.
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