Editing Metalloid (section)

==Periodic table territory==
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! colspan=12 style="text-align:center; background:{{element color|table title}}" | Distribution and recognition status<br>of elements classified as metalloids
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| style="line-height:100%;" | [[Alkali metal|<small>1</small>]]
| [[Alkaline earth metal|<small>2</small>]]
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| [[Group 12 element|<small>12</small>]]
| [[Boron group|<small>13</small>]]
| [[Carbon group|<small>14</small>]]
| [[Pnictogen|<small>15</small>]]
| [[Chalcogen|<small>16</small>]]
| [[Halogen|<small>17</small>]]
| [[Noble gas|<small>18</small>]]
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| style="width:2.5em; background:#faebd7; border-right:1px solid black; border-top:1px solid black; border-left:1px solid black; border-bottom:2px solid grey" | <big>[[Hydrogen|H]]</big>
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| style="width:1em;" | &nbsp;
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| style="width:2.5em; border-right:1px solid black; border-bottom:1px solid black"|&nbsp;
| style="width:2.5em; border-right:1px solid black; border-top:1px solid black; border-bottom:1px solid black" | <big>He</big>
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| style="border-right:1px solid black; border-top:2px solid grey; border-left:1px solid black; border-bottom:1px solid black" | <big>Li</big>
| style="background:#faebd7; border-right:2px solid grey; border-top:2px solid grey; border-bottom:1px solid black" | <big>[[Beryllium|Be]]</big>
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| style="background:#dcdcdc; border-right:1px solid black; border-left:2px solid grey; border-bottom:2px solid grey" | <big>[[Boron|B]]</big>
| style="background:#dcdcdc; border-right:1px solid black; border-bottom:1px solid black" | <big>[[Carbon|C]]</big>
| style="background:#f0f8ff; border-right:1px solid black; border-bottom:1px solid black" | <big>[[Nitrogen|N]]</big>
| style="border-right:1px solid black; border-bottom:1px solid black" | <big>O</big>
| style="border-right:1px solid black; border-bottom:1px solid black" | <big>F</big>
| style="border-right:1px solid black; border-bottom:1px solid black" | <big>Ne</big>
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| style="border-right:1px solid black; border-left:1px solid black; border-bottom:1px solid black" | <big>Na</big>
| style="border-right:1px solid black; border-bottom:1px solid black" | <big>Mg</big>
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| style="border-right:1px solid black; border-bottom:1px solid black" |
| style="background:#dcdcdc; border-right:2px solid grey; border-top:2px solid grey; border-bottom:1px solid black" | <big>[[Aluminium|Al]]</big>
| style="background:#dcdcdc; border-right:1px solid black; border-left:2px solid grey; border-bottom:2px solid grey" | <big>[[Silicon|Si]]</big>
| style="background:#faebd7; border-right:1px solid black; border-bottom:1px solid black" | <big>[[Phosphorus|P]]</big>
| style="background:#faebd7; border-right:1px solid black; border-bottom:1px solid black" | <big>[[Sulfur|S]]</big>
| style="border-right:1px solid black; border-bottom:1px solid black" | <big>Cl</big>
| style="border-right:1px solid black; border-bottom:1px solid black" | <big>Ar</big>
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| style="border-right:1px solid black; border-left:1px solid black; border-bottom:1px solid black" | <big>K</big>
| style="border-right:1px solid black; border-bottom:1px solid black" | <big>Ca</big>
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| style="background:#f0f8ff; border-right:1px solid black; border-bottom:1px solid black" | <big>[[Zinc|Zn]]</big>
| style="background:#faebd7; border-right:1px solid black; border-bottom:1px solid black" | <big>[[Gallium|Ga]]</big>
| style="background:#dcdcdc; border-right:2px solid grey; border-top:2px solid grey; border-bottom:1px solid black" | <big>[[Germanium|Ge]]</big>
| style="background:#dcdcdc; border-right:1px solid black; border-left:2px solid grey; border-bottom:2px solid grey" | <big>[[Arsenic|As]]</big>
| style="background:#dcdcdc; border-right:1px solid black; border-bottom:1px solid black" | <big>[[Selenium|Se]]</big>
| style="border-right:1px solid black; border-bottom:1px solid black" | <big>Br</big>
| style="border-right:1px solid black; border-bottom:1px solid black" | <big>Kr</big>
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| style="border-right:1px solid black; border-left:1px solid black; border-bottom:1px solid black" | <big>Rb</big>
| style="border-right:1px solid black; border-bottom:1px solid black" | <big>Sr</big>
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| style="border-right:1px solid black; border-bottom:1px solid black" | <big>Cd</big>
| style="border-right:1px solid black; border-bottom:1px solid black" | <big>In</big>
| style="background:#faebd7; border-right:1px solid black; border-bottom:1px solid black" | <big>[[Tin|Sn]]</big>
| style="background:#dcdcdc; border-right:2px solid grey; border-top:2px solid grey; border-bottom:1px solid black" | <big>[[Antimony|Sb]]</big>
| style="background:#dcdcdc; border-right:1px solid black; border-left:2px solid grey; border-bottom:2px solid grey" | <big>[[Tellurium|Te]]</big>
| style="background:#faebd7; border-right:1px solid black; border-bottom:1px solid black" | <big>[[Iodine|I]]</big>
| style="border-right:1px solid black; border-bottom:1px solid black" | <big>Xe</big>
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| style="border-right:1px solid black; border-left:1px solid black; border-bottom:1px solid black" | <big>Cs</big>
| style="border-right:1px solid black; border-bottom:1px solid black" | <big>Ba</big>
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| style="border-right:1px solid black; border-bottom:1px solid black" | <big>Hg</big>
| style="border-right:1px solid black; border-bottom:1px solid black" | <big>Tl</big>
| style="background:#faebd7; border-right:1px solid black; border-bottom:1px solid black" | <big>[[Lead|Pb]]</big>
| style="background:#faebd7; border-right:1px solid black; border-bottom:1px solid black" | <big>[[Bismuth|Bi]]</big>
| style="background:#dcdcdc; border-right:2px solid grey; border-top:2px solid grey; border-bottom:1px solid black" | <big>[[Polonium|Po]]</big>
| style="background:#dcdcdc; border-right:1px solid black; border-left:2px solid grey; border-bottom:2px solid grey" | <big>[[Astatine|At]]</big>
| style="background:#f0f8ff; border-right:1px solid black; border-bottom:1px solid black" | <big>[[Radon|Rn]]</big>
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| style="border-right:1px solid black; border-left:1px solid black; border-bottom:1px solid black" | <big>Fr</big>
| style="border-right:1px solid black; border-bottom:1px solid black" | <big>Ra</big>
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| style="border-right:1px solid black; border-bottom:1px solid black" | <big>Cn</big>
| style="border-right:1px solid black; border-bottom:1px solid black" | <big>Nh</big>
| style="background:#faebd7; border-right:1px solid black; border-bottom:1px solid black" | <big>[[Flerovium|Fl]]</big>
| style="background:#faebd7; border-right:1px solid black; border-bottom:1px solid black" | <big>[[Moscovium|Mc]]</big>
| style="background:#faebd7; border-right:1px solid black; border-bottom:1px solid black" | <big>[[Livermorium|Lv]]</big>
| style="background:#faebd7; border-right:2px solid grey; border-top:2px solid grey; border-bottom:1px solid black" | <big>[[Tennessine|Ts]]</big>
| style="border-right:1px solid black; border-left:2px solid grey; border-bottom:1px solid black" | <big>Og</big>
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| colspan="10" style="text-align:center; font-size:20%" |&nbsp;
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| colspan=10 style="padding-left:0.5em; background:#f8f8f8;" |
{{legend |#dcdcdc |Commonly (93%) to rarely (9%) recognised as a<br>metalloid: B, C, Al, Si, Ge, As, Se, Sb, Te, Po, At}}
{{legend |#faebd7 |Very rarely (1–5%): H, Be, P, S, Ga, Sn, I, Pb, Bi, Fl, Mc, Lv, Ts}}
{{legend |#f0f8ff |Sporadically: N, Zn, Rn}}
<div class="legend"><span style="display:inline-block; width:1.5em; height:1.5em; margin:1px 0; border:0px; border-top:4px solid grey; border-right:4px solid grey; background-color:#e8e8e8;">&nbsp;</span>&nbsp;[[Dividing line between metals and nonmetals|Metal–nonmetal dividing line]]: between {{nowrap|H and Li}}, {{nowrap|Be and B}}, {{nowrap|Al and Si}}, {{nowrap|Ge and As}}, {{nowrap|Sb and Te}}, {{nowrap|Po and At}}, and {{nowrap|Ts and Og}}</div><!-- code from {{legend}}, reduced -->
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| colspan="11" style="padding-left:0.5em; padding-right:2px"|
Periodic table extract showing groups 1&ndash;2 and 12&ndash;18, and a dividing line between metals and nonmetals. Percentages are median appearance frequencies in the [[list of metalloid lists]]. Sporadically recognised elements show that the metalloid net is sometimes cast very widely; although they do not appear in the list of metalloid lists, isolated references to their designation as metalloids can be found in the literature (as cited in this article).
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===Location===
Metalloids lie on either side of the [[dividing line between metals and nonmetals]]. This can be found, in varying configurations, on some [[periodic table]]s. Elements to the lower left of the line generally display increasing metallic behaviour; elements to the upper right display increasing nonmetallic behaviour.<ref name="Hamm 1969, p.653"/> When presented as a regular stairstep, elements with the highest [[critical point (thermodynamics)|critical temperature]] for their groups (Li, Be, Al, Ge, Sb, Po) lie just below the line.<ref>[[#Horvath1973|Horvath 1973, p.&nbsp;336]]</ref>

The diagonal positioning of the metalloids represents an exception to the observation that elements with similar properties tend to occur in vertical [[group (periodic table)|groups]].<ref name="Gray91">[[#Gray2009|Gray 2009, p.&nbsp;9]]</ref> A related effect can be seen in other [[diagonal relationship|diagonal similarities]] between some elements and their lower right neighbours, specifically lithium-magnesium, beryllium-aluminium, and boron-silicon. Rayner-Canham<ref name="Rayner2011">[[#Rayner2011|Rayner-Canham 2011]]</ref> has argued that these similarities extend to carbon-phosphorus, nitrogen-sulfur, and into three [[d-block]] series.

This exception arises due to competing horizontal and vertical trends in the [[nuclear charge]]. Going along a [[period (periodic table)|period]], the [[effective nuclear charge|nuclear charge]] increases with [[atomic number]] as do the number of electrons. The additional pull on outer electrons as nuclear charge increases generally outweighs the screening effect of having more electrons. With some irregularities, atoms therefore become smaller, ionization energy increases, and there is a gradual change in character, across a period, from strongly metallic, to weakly metallic, to weakly nonmetallic, to strongly nonmetallic elements.<ref>[[#Booth1972|Booth & Bloom 1972, p.&nbsp;426]]; [[#Cox2004|Cox 2004, pp.&nbsp;17, 18, 27–28]]; [[#Silberberg2006|Silberberg 2006, pp.&nbsp;305–13]]</ref> Going down a [[main-group element|main group]], the effect of increasing nuclear charge is generally outweighed by the effect of additional electrons being further away from the nucleus. Atoms generally become larger, ionization energy falls, and metallic character increases.<ref>[[#Cox2004|Cox 2004, pp.&nbsp;17–18, 27–28]]; [[#Silberberg2006|Silberberg 2006, pp.&nbsp;305–13]]</ref> The net effect is that the location of the metal–nonmetal transition zone shifts to the right in going down a group,<ref name=Gray91/> and analogous diagonal similarities are seen elsewhere in the periodic table, as noted.<ref>[[#Rodgers|Rodgers 2011, pp.&nbsp;232–33; 240–41]]</ref>

===Alternative treatments===
Elements bordering the metal–nonmetal dividing line are not always classified as metalloids, noting a binary classification can facilitate the establishment of rules for determining bond types between metals and nonmetals.<ref name=roher>[[#Roher2001|Roher 2001, pp.&nbsp;4–6]]</ref> In such cases, the authors concerned focus on one or more attributes of interest to make their classification decisions, rather than being concerned about the marginal nature of the elements in question. Their considerations may or not be made explicit and may, at times, seem arbitrary.<ref name=Sharp1981/>{{refn|Jones (2010, pp. 169–71): "Though classification is an essential feature of all branches of science, there are always hard cases at the boundaries. The boundary of a class is rarely sharp…Scientists should not lose sleep over the hard cases. As long as a classification system is beneficial to economy of description, to structuring knowledge and to our understanding, and hard cases constitute a small minority, then keep it. If the system becomes less than useful, then scrap it and replace it with a system based on different shared characteristics."|group=n}} Metalloids may be grouped with metals;<ref>[[#Tyler1948|Tyler 1948, p.&nbsp;105]]; [[#Reilly2002|Reilly 2002, pp.&nbsp;5–6]]</ref> or regarded as nonmetals;<ref>[[#Hampel1976|Hampel & Hawley 1976, p.&nbsp;174]];</ref> or treated as a sub-category of nonmetals.<ref>[[#Goodrich1844|Goodrich 1844, p.&nbsp;264]]; [[#TheChemical1897|''The Chemical News'' 1897, p.&nbsp;189]]; [[#Hampel1976|Hampel & Hawley 1976, p.&nbsp;191]]; [[#Lewis1993|Lewis 1993, p.&nbsp;835]]; [[#Hérold2006|Hérold 2006, pp.&nbsp;149–50]]</ref>{{refn|1=Oderberg<ref>[[#Oderberg2007|Oderberg 2007, p.&nbsp;97]]</ref> argues on [[ontology|ontological]] grounds that anything not a metal is therefore a nonmetal, and that this includes semi-metals (i.e. metalloids).|group=n}} Other authors have suggested classifying some elements as metalloids "emphasizes that properties change gradually rather than abruptly as one moves across or down the periodic table".<ref name=brown>[[#Brown2006|Brown & Holme 2006, p.&nbsp;57]]</ref> Some periodic tables distinguish elements that are metalloids and display no formal dividing line between metals and nonmetals. Metalloids are instead shown as occurring in a diagonal band<ref>[[#Wiberg2001|Wiberg 2001, p.&nbsp;282]]; [[#Simple2005|Simple Memory Art c. 2005]]</ref> or diffuse region.<ref>[[#Chedd1969|Chedd 1969, pp.&nbsp;12–13]]</ref> The key consideration is to explain the context for the taxonomy in use.