Nonmetal, or non-metal, is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, every element in the periodic table can be termed either a metal or a nonmetal. (A few elements with intermediate properties are referred to as metalloids.)
The elements generally regarded as nonmetals are:
- In Group 1: hydrogen (H) (The only nonmetal in this group)
- In Group 14: carbon (C) (Again, the only nonmetal in this group)
- In Group 15: (the pnictogens): nitrogen (N), phosphorus (P)
- In Group 16: (the chalcogens): oxygen (O), sulfur (S), selenium (Se)
- In Group 17: (the halogens): fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At)
- All elements (with the possible exception of ununoctium) in Group 18 – the noble gases
There is no rigorous definition for the term "nonmetal" – it covers a general spectrum of behaviour. Common properties considered characteristic of a nonmetal include:
- poor conductors of heat and electricity when compared to metals
- they form acidic oxides (whereas metals generally form basic oxides)
- in solid form, they are dull and brittle, rather than metals which are lustrous, ductile or malleable
- usually have lower densities than metals
- they have significantly lower melting points and boiling points than metals (with the exception of carbon)
- non-metals have high electronegativity
Only eighteen elements in the periodic table are generally considered nonmetals, compared to over eighty metals, but nonmetals make up most of the crust, atmosphere and oceans of the earth. Bulk tissues of living organisms are composed almost entirely of nonmetals. Most nonmetals are monatomic noble gases or form diatomic molecules in their elemental state, unlike metals which (in their elemental state) do not form molecules at all.
Among the nonmetals, many possess metallic allotropes under high pressure, while some metals may exist in nonmetallic allotropes:
|Hydrogen||1||1||other nonmetal||Metallic hydrogen forms at 260–270 GPa at 295 K and converts back to molecular hydrogen at 200 GPa.1|
|Helium||1||18||noble gas||Metallic helium is predicted to occur around 100 Mbar (10 TPa) at low temperatures and 40 Mbar (4 TPa) at high temperatures.2|
|Boron||2||13||metalloid||Common allotropes of boron have bandgaps of approximately 2 eV, but a high-pressure superconducting phase occurs at 160 GPa and 250 GPa at 4 and 11 K.34|
|Carbon||2||14||other nonmetal||A metallic allotrope of carbon has been hypothesized to occur at 1.1 TPa.567|
|Nitrogen||2||15||other nonmetal||There has been some theoretical consideration of a high-pressure metallic allotrope.8 Despite a calculated transition at 100 GPa, experiments up to 180 GPa failed to detect this.5|
|Oxygen||2||16||other nonmetal||Metallic oxygen has been observed at pressures over 96 GPa, and is superconducting at low temperatures.9101112|
|Fluorine||2||17||halogen||The metallization pressure of solid fluorine is expected to exceed 200 GPa.13|
|Neon||2||18||noble gas||First-principle calculations estimate that the band gap of neon might close at 142 TPa and metallization may occur at 176 TPa.14|
|Aluminium||3||13||post-transition metal||The structure of clusters of aluminium atoms sandwiched among other elements can be extended to hypothesize a nonmetallic "β-aluminium" allotrope; it is not known whether it can physically exist.15|
|Silicon||3||14||metalloid||Under increasing pressure silicon transforms from a cubic diamond structure to a β-tin (11–12 GPa), primitive hexagonal (13–16 GPa), hexagonal-close-packed (37–40 GPa), and face-centered cubic phases (78 GPa). Three of these phases are metallic.1617|
|Phosphorus||3||15||other nonmetal||Two allotropes of phosphorus at atmospheric pressure have sometimes been called metallic – "α-metallic" (violet or Hittorf's phosphorus) and "β-metallic" or black phosphorus. Violet and black phosphorus have bandgaps of 1.5 and 0.34 eV, respectively.1819 Black phosphorus metallizes at 1.7 GPa by bandgap closure without a structural transition.5|
|Sulfur||3||16||other nonmetal||Sulfur undergoes transitions to two superconducting metal phases, at roughly 90 GPa and 200 GPa; the first of these has an incommensurate crystal structure.20|
|Chlorine||3||17||halogen||Chlorine was estimated to undergo transition to a metal at 67 GPa; this was confirmed, but at higher pressures.5|
|Argon||3||18||noble gas||As of 2009, metallization of argon, predicted to occur at very high pressures, has not been observed.21|
|Gallium||4||13||post-transition metal||The orthorhombic α-phase of gallium includes a short covalent bond between two of eight atoms of the unit cell, and has a "deep minimum in the electronic density of states at the Fermi energy"; thus it can be called a "metallic molecular crystal" or an "inorganic polymer". These properties are absent in the metallic Ga-II and β-Ga states.22|
|Germanium||4||14||metalloid||Germanium undergoes a semiconductor to metal transition at 11 GPa.5|
|Selenium||4||16||other nonmetal||Selenium undergoes a semiconductor to metal transition at 20 GPa.5|
|Bromine||4||17||halogen||Bromine undergoes a semiconductor to metal transition at 100 GPa.5|
|Krypton||4||18||noble gas||Metallization of krypton was predicted to occur at 316 GPa.23|
|Tin||5||14||post-transition metal||Of the two common allotropes of tin at room temperature and pressure, white tin (β-tin) is metallic, but gray tin (α-tin) is not. Gray tin is more stable at colder temperatures.|
|Tellurium||5||16||metalloid||Tellurium undergoes a semiconductor to metal transition at 4 GPa.5|
|Iodine||5||17||halogen||Iodine undergoes a semiconductor to metal transition at 17 GPa.5|
|Xenon||5||18||noble gas||Xenon undergoes a semiconductor to metal transition at 160 GPa.5|
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