Editing Nonmetal (section)

===Complications===

Adding complexity to the chemistry of the nonmetals are anomalies occurring in the first row of each [[periodic table block]]; non-uniform periodic trends; higher oxidation states; multiple bond formation; and property overlaps with metals.

====First row anomaly====
<div style="line-height:1px;">[[Image:1x1.png|link=|alt=A table with seven rows and ten columns. Rows are labeled on the left with a period number from 1 through 7. Columns are labeled on the bottom with a group number. Most cells represent a single chemical element and have two lines of information: the element's symbol on the top and its atomic number on the bottom. The table as a whole is divided into four rectangular areas separated from each other by narrow gaps. The first rectangle fills all seven rows of the first two columns. The rectangle is labeled "s-block" at the top and its two columns are labeled with group numbers "(1)" and "(2)" on the bottom. The cells in the first row - hydrogen and helium, with symbols H and He and atomic numbers 1 and 2 respectively - are both shaded red. The second rectangle fills the bottom two rows (periods 6 and 7) of the third column. Just above these cells is the label "f-block"; there is no group label on the bottom. The topmost cell - labeled "La-Yb" for elements 57-70 - is shaded green. The third rectangle fills the bottom four rows (periods 4 through 7) of the fourth column. Just above these cells is the label "d-block"; at the bottom is the label "(3-12)" for the group numbers of these elements. The topmost cell - labeled "Sc-Zn" for elements 21-30 - is shaded blue. The fourth and last rectangle fills the bottom six rows (periods 2 through 7) of the last six columns. Just above these cells is the label "p-block"; at the bottom are labels "(13)" through "(18) for the group numbers of these elements. The cells in the topmost row - for the elements boron (B,5), carbon (C,6), nitrogen (N,7), oxygen (O,8), fluorine (Fl,9), and neon (Ne,10) - are shaded yellow. Bold lines encircle the cells of the nonmetals - the top two cells on the left and 21 cells in the upper right of the table.]]</div>
{| class=" floatright" style="border-collapse:collapse; text-align:center;font-size:80%;line-height:1.1;margin-top:1.2em;"
| colspan=14 style="padding-bottom:3px;border:none;text-align:center;font-size:105%" | '''Condensed periodic table highlighting<br>the first row of each block: {{color box|{{element color|s-block}}|s}} {{color box|{{element color|p-block}}|p}} {{color box|{{element color|d-block}}|d}} and {{color box|{{element color|f-block}}|f}}'''
|-
| colspan=1 | '''Period'''
| colspan=2 | '''{{nowrap|s-block}}'''
| rowspan=9 style="padding:1px;" |
| colspan=1 |
| rowspan=9 style="padding:1px;" |
| colspan=1 |
| rowspan=9 style="padding:1px;" |
| colspan=6 |
|-
| '''1'''
| style="border:solid black;border-width:2px 1px 2px 2px;background-color:{{element color|s-block}};" | H <br>1
| style="border:solid black;border-width:2px 2px 2px 1px;background-color:{{element color|s-block}};" | He<br>2
| 
| 
| colspan=6 | <br>'''p-block'''
|-
| '''2'''
| style="border:solid black;border-width:1px 1px 1px 1px;" | Li<br>3
| style="border:solid black;border-width:1px 1px 1px 1px;" | Be<br>4
|
|
| style="border:solid black;border-width:2px 1px 2px 2px;background-color:{{element color|p-block}};" | B <br>5
| style="border:solid black;border-width:2px 1px 1px 1px;background-color:{{element color|p-block}};" | C <br>6
| style="border:solid black;border-width:2px 1px 1px 1px;background-color:{{element color|p-block}};" | N <br>7
| style="border:solid black;border-width:2px 1px 1px 1px;background-color:{{element color|p-block}};" | O <br>8
| style="border:solid black;border-width:2px 1px 1px 1px;background-color:{{element color|p-block}};" | F <br>9
| style="border:solid black;border-width:2px 2px 1px 1px;background-color:{{element color|p-block}};" | Ne<br>10
|-
| '''3'''
| style="border:solid black;border-width:1px 1px 1px 1px;" | Na<br>11
| style="border:solid black;border-width:1px 1px 1px 1px;" | Mg<br>12
|
| <br>'''{{nowrap|d-block}}'''
| style="border:solid black;border-width:1px 1px 1px 1px;" | Al<br>13
| style="border:solid black;border-width:1px 1px 1px 2px;" | Si<br>14
| style="border:solid black;border-width:1px 1px 1px 1px;" | P <br>15
| style="border:solid black;border-width:1px 1px 1px 1px;" | S <br>16
| style="border:solid black;border-width:1px 1px 1px 1px;" | Cl<br>17
| style="border:solid black;border-width:1px 2px 1px 1px;" | Ar<br>18
|-
| '''4'''
| style="border:solid black;border-width:1px 1px 1px 1px;" | K <br>19
| style="border:solid black;border-width:1px 1px 1px 1px;" | Ca<br>20
|
| style="border:solid black;border-width:1px 1px 1px 1px;background-color:{{element color|d-block}};" | Sc-Zn<br>21-30
| style="border:solid black;border-width:1px 1px 1px 1px;" | Ga<br>31
| style="border:solid black;border-width:1px 1px 2px 2px;" | Ge<br>32
| style="border:solid black;border-width:1px 1px 1px 1px;" | As<br>33
| style="border:solid black;border-width:1px 1px 1px 1px;" | Se<br>34
| style="border:solid black;border-width:1px 1px 1px 1px;" | Br<br>35
| style="border:solid black;border-width:1px 2px 1px 1px;" | Kr<br>36
|-
| '''5'''
| style="border:solid black;border-width:1px 1px 1px 1px;" | Rb<br>37
| style="border:solid black;border-width:1px 1px 1px 1px;" | Sr<br>38
| <br>'''{{nowrap|f-block}}'''
| style="border:solid black;border-width:1px 1px 1px 1px;" | Y-Cd<br>39-48
| style="border:solid black;border-width:1px 1px 1px 1px;" | In<br>49
| style="border:solid black;border-width:1px 1px 1px 1px;" | Sn<br>50
| style="border:solid black;border-width:1px 1px 2px 2px;" | Sb<br>51
| style="border:solid black;border-width:1px 1px 2px 1px;" | Te<br>52
| style="border:solid black;border-width:1px 1px 2px 1px;" | I <br>53
| style="border:solid black;border-width:1px 2px 1px 1px;" | Xe<br>54
|-
| '''6'''
| style="border:solid black;border-width:1px 1px 1px 1px;" | Cs<br>55
| style="border:solid black;border-width:1px 1px 1px 1px;" | Ba<br>56
| style="border:solid black;border-width:1px 1px 1px 1px;background-color:{{element color|f-block}};" | La-Yb<br>57-70
| style="border:solid black;border-width:1px 1px 1px 1px;" | ''Lu-Hg<br>71-80''
| style="border:solid black;border-width:1px 1px 1px 1px;" | Tl<br>81
| style="border:solid black;border-width:1px 1px 1px 1px;" | Pb<br>82
| style="border:solid black;border-width:1px 1px 1px 1px;" | Bi<br>83
| style="border:solid black;border-width:1px 1px 1px 1px;" | Po<br>84
| style="border:solid black;border-width:1px 1px 1px 1px;" | At<br>85
| style="border:solid black;border-width:1px 2px 2px 2px;" | Rn<br>86
|-
| '''7'''
| style="border:solid black;border-width:1px 1px 1px 1px;" | Fr<br>87
| style="border:solid black;border-width:1px 1px 1px 1px;" | Ra<br>88
| style="border:solid black;border-width:1px 1px 1px 1px;" | Ac-No<br>89-102
| style="border:solid black;border-width:1px 1px 1px 1px;" | Lr-Cn<br>103-112
| style="border:solid black;border-width:1px 1px 1px 1px;" | Nh<br>113
| style="border:solid black;border-width:1px 1px 1px 1px;" | Fl<br>114
| style="border:solid black;border-width:1px 1px 1px 1px;" | Mc<br>115
| style="border:solid black;border-width:1px 1px 1px 1px;" | Lv<br>116
| style="border:solid black;border-width:1px 1px 1px 1px;" | Ts<br>117
| style="border:solid black;border-width:1px 1px 1px 1px;" | Og<br>118
|-
| ''Group''
| ''(1)''
| ''(2)''
|
| ''(3-12)''
| ''(13)''
| ''(14)''
| ''(15)''
| ''(16)''
| ''(17)''
| ''(18)''
|-
| colspan=14 style="border:none;"|
|-
| colspan=14 style="border:none; text-align:Center;font-size:105%;"| The [[Kainosymmetry|first-row anomaly]] strength by block is '''s''' >> '''p''' > '''d''' > '''f'''.<ref>[[#Jensen|Jensen 1986, p. 506]]</ref>{{efn|Helium is shown above beryllium for electron configuration consistency purposes; as a noble gas it is usually placed above neon, in group 18.}}
|}
Starting with hydrogen, the [[Kainosymmetry|first row anomaly]] primarily arises from the electron configurations of the elements concerned. Hydrogen is notable for its diverse bonding behaviors. It most commonly forms covalent bonds, but it can also lose its single electron in an [[aqueous solution]], leaving behind a bare proton with high polarizing power.<ref>[[#Lee|Lee 1996, p. 240]]</ref> Consequently, this proton can attach itself to the lone electron pair of an oxygen atom in a water molecule, laying the foundation for [[acid-base chemistry]].<ref>[[#Greenwood|Greenwood & Earnshaw 2002, p. 43]]</ref> Moreover, a hydrogen atom in a molecule can form a [[hydrogen bonding|second, albeit weaker, bond]] with an atom or group of atoms in another molecule. Such bonding, "helps give [[snowflake]]s their hexagonal symmetry, binds [[DNA]] into a [[double helix]]; shapes the three-dimensional forms of [[protein]]s; and even raises water's boiling point high enough to make a decent cup of tea."<ref>[[#Cressey|Cressey 2010]]</ref>

Hydrogen and helium, as well as boron through neon, have small atomic radii. The ionization energies and electronegativities among these elements are higher than the [[periodic trends]] would otherwise suggest.

While it would normally be expected, on electron configuration consistency grounds, that hydrogen and helium would be placed atop the s-block elements, the significant first row anomaly shown by these two elements justifies alternative placements. Hydrogen is occasionally positioned above fluorine, in group 17, rather than above lithium in group 1. Helium is almost always placed above neon, in group 18, rather than above beryllium in group 2.<ref>[[#Petruševski|Petruševski & Cvetković 2018]]; [[#Grochala|Grochala 2018]]</ref>

====Secondary periodicity====
[[File:EN values of chalcogens.png|thumb|upright=0.8|Electronegativity values of the group 16 [[chalcogen]] elements showing a W-shaped alternation or secondary periodicity going down the group|alt=A graph with a vertical electronegativity axis and a horizontal atomic number axis. The five elements plotted are {{abbr|O|oxygen}}, {{abbr|S|sulfur}}, {{abbr|Se|selenium}}, {{abbr|Te|tellurium}} and {{abbr|Po|polonium}}. The electronegativity of {{abbr|Se|selenium}} looks too high, and causes a bump in what otherwise be a smooth curve.]]
An alternation in certain periodic trends, sometimes referred to as [[Periodic table#Atomic radius|secondary periodicity]], becomes evident when descending groups 13 to 15, and to a lesser extent, groups 16 and 17.<ref>[[#Kneen|Kneen, Rogers & Simpson 1972, pp. 226, 360]]; [[#Siekierski|Siekierski & Burgess 2002, pp. 52, 101, 111, 124, 194]]</ref>{{efn|The net result is an even-odd difference between periods (except in the [[s-block]]): elements in even periods have smaller atomic radii and prefer to lose fewer electrons, while elements in odd periods (except the first) differ in the opposite direction. Many properties in the p-block then show a zigzag rather than a smooth trend along the group. For example, phosphorus and antimony in odd periods of group 15 readily reach the +5 oxidation state, whereas nitrogen, arsenic, and bismuth in even periods prefer to stay at +3.<ref>[[#Scerri2020|Scerri 2020, pp. 407–420]]</ref>}} Immediately after the first row of [[Block (periodic table)#d-block|d-block]] metals, from scandium to zinc, the 3d electrons in the [[Block (periodic table)#p-block|p-block]] elements—specifically, gallium (a metal), germanium, arsenic, selenium, and bromine—prove less effective at [[shielding effect|shielding]] the increasing positive nuclear charge.

The Soviet chemist {{Interlanguage link|Shchukarev|2=ru|3=Щукарев, Сергей Александрович|preserve=1}} gives two more tangible examples:<ref>[[#Shchukarev|Shchukarev 1977, p. 229]]</ref>
:<span style="font-size:95%">"The toxicity of some arsenic compounds, and the absence of this property in analogous compounds of phosphorus [P] and antimony [Sb]; and the ability of [[selenic acid]] [{{chem2|H2SeO4}}] to bring metallic gold [Au] into solution, and the absence of this property in sulfuric [[sulfuric acid|[{{chem2|H2SO4}}]]] and [[telluric acid|[{{chem2|H2TeO4}}]]] acids."</span>

====Higher oxidation states====
:''Roman numerals such as III, V and VIII denote oxidation states''

Some nonmetallic elements exhibit [[oxidation state]]s that deviate from those predicted by the octet rule, which typically results in an oxidation state of –3 in group 15, –2 in group 16, –1 in group 17, and 0 in group 18. Examples include [[ammonia]] NH<sub>3</sub>, [[hydrogen sulfide]] H<sub>2</sub>S, [[hydrogen fluoride]] HF, and elemental xenon Xe. Meanwhile, the maximum possible oxidation state increases from +5 in [[pnictogen|group 15]], to +8 in [[noble gas|group 18]]. The +5 oxidation state is observable from period 2 onward, in compounds such as [[nitric acid]] HN(V)O<sub>3</sub> and [[phosphorus pentafluoride]] PCl<sub>5</sub>.{{efn|Oxidation states do not reflect the actual net charge of atoms in molecules or ions, they represents the valence which refers more to how many bonds there are. For instance carbon typically has a valence of +4, but that only means that it forms three bonds. Electronegative elements such as fluorine are conventionally associated with negative valence, while electropositive ones have positive valence.}} [[Oxidation state#List of oxidation states of the elements|Higher oxidation states]] in later groups emerge from period 3 onwards, as seen in [[sulfur hexafluoride]] SF<sub>6</sub>, [[iodine heptafluoride]] IF<sub>7</sub>, and [[xenon tetroxide|xenon(VIII) tetroxide]] XeO<sub>4</sub>. For heavier nonmetals, their larger atomic radii and lower electronegativity values enable the formation of compounds with higher oxidation numbers, supporting higher bulk [[coordination number]]s.<ref name="Cox" />

====Multiple bond formation====
[[File:Pentazenium.png|thumb|right|alt=A chain of five N's in a wing shape|Molecular structure of [[pentazenium]], a homopolyatomic cation of nitrogen with the formula N<sub>5</sub><sup>+</sup> and structure N−N−N−N−N.<ref>[[#Vij|Vij et al. 2001]]</ref>]]Period 2 nonmetals, particularly carbon, nitrogen, and oxygen, show a propensity to form multiple bonds. The compounds formed by these elements often exhibit unique [[stoichiometries]] and structures, as seen in the various nitrogen oxides,<ref name="Cox">[[#Cox2004|Cox 2004, p. 146]]</ref> which are not commonly found in elements from later periods.

==== Property overlaps ====
While certain elements have traditionally been classified as nonmetals and others as metals, some overlapping of properties occurs. Writing early in the twentieth century, by which time the era of modern chemistry had been well-established<ref>[[#Dorsey|Dorsey 2023, pp. 12–13]]</ref> (although not as yet more precise [[quantum chemistry]]) Humphrey<ref>[[#Humphrey|Humphrey 1908]]</ref> observed that:
:<span style="font-size:95%">... these two groups, however, are not marked off perfectly sharply from each other; some nonmetals resemble metals in certain of their properties, and some metals approximate in some ways to the non-metals.</span>

[[Image:brown-boron.jpg|thumb|right|alt=An open glass jar with a brown powder in it|Boron (here in its less stable amorphous form) shares some similarities with metals{{efn|Greenwood<ref>[[#Greenwood2001|Greenwood 2001, p.&nbsp;2057]]</ref> commented that: "The extent to which metallic elements mimic boron (in having fewer electrons than orbitals available for bonding) has been a fruitful cohering concept in the development of metalloborane chemistry&nbsp;... Indeed, metals have been referred to as "honorary boron atoms" or even as "flexiboron atoms". The converse of this relationship is clearly also valid."}}]]
Examples of metal-like properties occurring in nonmetallic elements include:
* Silicon has an electronegativity (1.9) comparable with metals such as cobalt (1.88), copper (1.9), nickel (1.91) and silver (1.93);<ref name="AylwardEN"/>
* The electrical conductivity of graphite exceeds that of some metals;{{efn|For example, the conductivity of graphite is 3 × 10<sup>4</sup> S•cm<sup>−1.</sup><ref name=Bog/> whereas that of [[manganese]] is 6.9 × 10<sup>3</sup> S•cm<sup>−1</sup>.<ref>[[#Desai|Desai, James & Ho 1984, p.&nbsp;1160]]</ref>}}
* Selenium can be drawn into a wire;<ref name="ReferenceF"/>
* Radon is the most metallic of the noble gases and begins to show some [[cation]]ic behavior, which is unusual for a nonmetal;<ref>[[#Stein1983|Stein 1983, p. 165]]</ref> and
* In extreme conditions, just over half of nonmetallic elements can form homopolyatomic cations.{{efn|A homopolyatomic cation consists of two or more atoms of the same element bonded together and carrying a positive charge, for example, N<sub>5</sub><sup>+</sup>, O<sub>2</sub><sup>+</sup> and Cl<sub>4</sub><sup>+</sup>. This is unusual behavior for nonmetals since cation formation is normally associated with metals, and nonmetals are normally associated with anion formation. Homopolyatomic cations are further known for carbon, phosphorus, antimony, sulfur, selenium, tellurium, bromine, iodine and xenon.<ref>[[#Engesser|Engesser & Krossing 2013, p. 947]]</ref>}}

Examples of nonmetal-like properties occurring in metals are:
*[[Tungsten]] displays some nonmetallic properties, sometimes being brittle, having a high electronegativity, and forming only anions in aqueous solution,<ref>[[#S&P|Schweitzer & Pesterfield 2010, p. 305]]</ref> and predominately acidic oxides.<ref name="Porterfield"/><ref>[[#Rieck|Rieck 1967, p. 97]]: Tungsten trioxide dissolves in [[hydrofluoric acid]] to give an [[oxyfluoride]] [[Coordination complex|complex]].</ref>
*[[Gold]], the "king of metals" has the highest [[electrode potential]] among metals, suggesting a preference for gaining rather than losing electrons. Gold's ionization energy is one of the highest among metals, and its electron affinity and electronegativity are high, with the latter exceeding that of some nonmetals. It forms the Au<sup>–</sup> auride anion and exhibits a tendency to bond to itself, behaviors which are unexpected for metals. In aurides (MAu, where M = Li–Cs), gold's behavior is similar to that of a halogen.<ref>[[#Wiberg|Wiberg 2001, p. 1279]]</ref> The reason for this is that gold has a large enough nuclear potential that the electrons have to be considered with [[Relativistic quantum mechanics|relativistic]] effects included, which changes some of the properties.<ref>{{Cite journal |last=Pyper |first=N. C. |date=2020-09-18 |title=Relativity and the periodic table |url=https://royalsocietypublishing.org/doi/10.1098/rsta.2019.0305 |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |language=en |volume=378 |issue=2180 |pages=20190305 |doi=10.1098/rsta.2019.0305 |pmid=32811360 |bibcode=2020RSPTA.37890305P |issn=1364-503X}}</ref>

A relatively recent development involves certain compounds of heavier p-block elements, such as silicon, phosphorus, germanium, arsenic and antimony, exhibiting behaviors typically associated with [[coordination compound|transition metal complexes]]. This is linked to a small energy gap between their [[HOMO and LUMO|filled and empty]] [[molecular orbitals]], which are the regions in a molecule where electrons reside and where they can be available for chemical reactions. In such compounds, this allows for unusual reactivity with small molecules like hydrogen (H<sub>2</sub>), [[ammonia]] (NH<sub>3</sub>), and [[ethylene]] (C<sub>2</sub>H<sub>4</sub>), a characteristic previously observed primarily in transition metal compounds. These reactions may open new avenues in [[catalyst|catalytic]] applications.<ref>[[#Power|Power 2010]]; [[#Crow|Crow 2013]]; [[#Weetman|Weetman & Inoue 2018]]</ref>