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|Common [[allotropes of boron]] have [[bandgap]]s of approximately 2 eV, but a [[Allotropes_of_boron#High-pressure_superconducting_phase|high-pressure superconducting phase]] occurs at 160 GPa and 250 GPa at 4 and 11 K.<ref>{{cite journal| author = M. I. Eremets et al. |title = Superconductivity in Boron| doi=10.1126/science.1062286 |journal = Science |volume = 293 |year = 2001 |page = 272| pmid = 11452118| issue = 5528|bibcode = 2001Sci...293..272E| pages = 272–4 }}</ref><ref>{{cite journal| title = High-pressure metallic phases of boron| author =C. Mailhiot, J. B. Grant, and A. K. McMahan| journal = Phys. Rev. B |volume = 42| page = 9033| year = 1990| doi = 10.1103/PhysRevB.42.9033|bibcode = 1990PhRvB..42.9033M| issue = 14 }}</ref> |
|Common [[allotropes of boron]] have [[bandgap]]s of approximately 2 eV, but a [[Allotropes_of_boron#High-pressure_superconducting_phase|high-pressure superconducting phase]] occurs at 160 GPa and 250 GPa at 4 and 11 K.<ref>{{cite journal| author = M. I. Eremets et al. |title = Superconductivity in Boron| doi=10.1126/science.1062286 |journal = Science |volume = 293 |year = 2001 |page = 272| pmid = 11452118| issue = 5528|bibcode = 2001Sci...293..272E| pages = 272–4 }}</ref><ref>{{cite journal| title = High-pressure metallic phases of boron| author =C. Mailhiot, J. B. Grant, and A. K. McMahan| journal = Phys. Rev. B |volume = 42| page = 9033| year = 1990| doi = 10.1103/PhysRevB.42.9033|bibcode = 1990PhRvB..42.9033M| issue = 14 }}</ref> |
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|A [[Allotropes_of_carbon#Other_possible_forms|metallic allotrope of carbon]] has been hypothesized to occur at extremely high pressures.<ref>{{cite journal|url=http://pubs.acs.org/doi/abs/10.1021/ja00352a049|title=Hypothetical metallic allotrope of carbon|author=Roald Hoffmann, Timothy Hughbanks, Miklos Kertesz, Peter H. Bird|journal=J. Am. Chem. Soc.|volume=105|issue=14|pages=4831–4832|doi=DOI: 10.1021/ja00352a049|date=1983-07}}</ref><ref>{{cite journal|doi=10.1073/pnas.0510489103|year=2006|month=Jan|author=Correa, Aa; Bonev, Sa; Galli, G|title=Carbon under extreme conditions: phase boundaries and electronic properties from first-principles theory|volume=103|issue=5|pages=1204–8|issn=0027-8424|pmid=16432191|pmc=1345714|journal=Proceedings of the National Academy of Sciences of the United States of America|url=http://www.pnas.org/cgi/pmidlookup?view=long&pmid=16432191|bibcode = 2006PNAS..103.1204C }}</ref> |
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Revision as of 17:26, 24 February 2012
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:
- hydrogen
- In Group 14: carbon (C)
- In Group 15 (the pnictogens): nitrogen (N), phosphorus (P)
- In Group 16 (the chalcogens): oxygen (O), sulfur (S), selenium (Se)
- All elements (with the possible exception of ununseptium) in Group 17 - the halogens
- 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.
Metallic allotropes
Among the nonmetals, many possess metallic allotropes under high pressure, while some metals may exist in nonmetallic allotropes:
| Element | Period | Group | Classification | Allotropes |
|---|---|---|---|---|
| Hydrogen | 1 | 1 | 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 at low temperatures and 40 Mbar 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.[3][4] |
| Carbon | 2 | 14 | nonmetal | A metallic allotrope of carbon has been hypothesized to occur at extremely high pressures.[5][6] |
See also
- ^ Eremets, M.I.; Troyan, I.A. (2011). "Conductive dense hydrogen". Nature Materials. doi:10.1038/nmat3175.
- ^ David J. Stevenson (2008-08-06). "Metallic helium in massive planets". PNAS.
{{cite journal}}: Cite journal requires|journal=(help) - ^ M. I. Eremets; et al. (2001). "Superconductivity in Boron". Science. 293 (5528): 272. Bibcode:2001Sci...293..272E. doi:10.1126/science.1062286. PMID 11452118.
{{cite journal}}: Explicit use of et al. in:|author=(help); More than one of|pages=and|page=specified (help) - ^ C. Mailhiot, J. B. Grant, and A. K. McMahan (1990). "High-pressure metallic phases of boron". Phys. Rev. B. 42 (14): 9033. Bibcode:1990PhRvB..42.9033M. doi:10.1103/PhysRevB.42.9033.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Roald Hoffmann, Timothy Hughbanks, Miklos Kertesz, Peter H. Bird (1983-07). "Hypothetical metallic allotrope of carbon". J. Am. Chem. Soc. 105 (14): 4831–4832. doi:DOI: 10.1021/ja00352a049.
{{cite journal}}: Check|doi=value (help); Check date values in:|date=(help)CS1 maint: multiple names: authors list (link) - ^ Correa, Aa; Bonev, Sa; Galli, G (2006). "Carbon under extreme conditions: phase boundaries and electronic properties from first-principles theory". Proceedings of the National Academy of Sciences of the United States of America. 103 (5): 1204–8. Bibcode:2006PNAS..103.1204C. doi:10.1073/pnas.0510489103. ISSN 0027-8424. PMC 1345714. PMID 16432191.
{{cite journal}}: Unknown parameter|month=ignored (help)CS1 maint: multiple names: authors list (link)
