Editing Sodium (section)

===Physical===
[[File:Na-D-sodium D-lines-589nm.jpg|thumb|left|[[Emission spectrum]] for sodium, showing the [[Fraunhofer lines|D line]]]]
Sodium at [[standard temperature and pressure]] is a soft silvery metal that combines with oxygen in the air, forming [[sodium oxide]]s.  Bulk sodium is usually stored in oil or an inert gas. Sodium metal can be easily cut with a knife. It is a good conductor of electricity and heat.

The melting (98&nbsp;°C) and boiling (883&nbsp;°C) points of sodium are lower than those of lithium but higher than those of the heavier alkali metals potassium, rubidium, and caesium, following periodic trends down the group.<ref>{{cite encyclopedia|url=http://www.encyclopedia.com/science-and-technology/chemistry/compounds-and-elements/alkali-metals|title="Alkali Metals." Science of Everyday Things|encyclopedia=Encyclopedia.com|access-date=15 October 2016|archive-date=17 October 2016|archive-url=https://web.archive.org/web/20161017234708/http://www.encyclopedia.com/science-and-technology/chemistry/compounds-and-elements/alkali-metals|url-status=live}}</ref> These properties change dramatically at elevated pressures: at 1.5&nbsp;[[Bar (unit)|Mbar]], the color changes from silvery metallic to black; at 1.9&nbsp;Mbar the material becomes transparent with a red color; and at 3 Mbar, sodium is a clear and transparent solid. All of these high-pressure [[allotrope]]s are insulators and [[electride]]s.<ref>{{cite journal|last1=Gatti|first1=M.|last2=Tokatly|first2=I.|last3=Rubio|first3=A.|date=2010|title=Sodium: A Charge-Transfer Insulator at High Pressures|journal=[[Physical Review Letters]]|volume=104|issue=21|page=216404|pmid=20867123|doi=10.1103/PhysRevLett.104.216404|bibcode=2010PhRvL.104u6404G|arxiv = 1003.0540 |s2cid=18359072}}</ref>

[[File:Flametest--Na.swn.jpg|thumb|left|upright|A positive [[flame test]] for sodium has a bright yellow color.]]
In a [[flame test]], sodium and its compounds glow yellow<ref>{{cite book|last=Schumann|first=Walter|title=Minerals of the World|date=5 August 2008|publisher=Sterling|isbn=978-1-4027-5339-8|edition=2nd|page=28|oclc=637302667}}</ref> because the excited [[atomic orbital|3s]] electrons of sodium emit a [[photon]] when they fall from 3p to 3s; the wavelength of this photon corresponds to the [[D2 line|D line]] at about 589.3&nbsp;nm. [[Spin-orbit interaction]]s involving the electron in the 3p orbital split the D line into two, at 589.0 and 589.6&nbsp;nm; [[hyperfine structure]]s involving both orbitals cause many more lines.<ref name="Citron-PRL-1977">{{cite journal|last1=Citron|first1=M. L.|last2=Gabel|first2=C.|last3=Stroud|first3=C.|date=1977|title=Experimental Study of Power Broadening in a Two-Level Atom|journal=Physical Review A|volume=16|doi=10.1103/PhysRevA.16.1507|pages=1507–1512|issue=4|bibcode=1977PhRvA..16.1507C|last4=Stroud|first4=C.}}</ref>