Editing Potassium (section)

===Physical===
[[File:FlammenfärbungK.png|thumb|upright=0.5|The [[flame test]] of potassium.]]
Potassium is the second least dense metal after [[lithium]]. It is a soft solid with a low [[melting point]], and can be easily cut with a knife. Potassium is silvery in appearance, but it begins to tarnish toward gray immediately on exposure to air.<ref name="g76">[[#Greenwood|Greenwood]], p. 76</ref> In a [[flame test]], potassium and its compounds emit a [[Lilac (color)|lilac color]] with a peak emission wavelength of 766.5 nanometers.<ref>[[#Greenwood|Greenwood]], p. 75</ref><!-- Potassium concentration in solution is commonly determined using [[photoelectric flame photometer|flame photometry]], [[atomic absorption spectrophotometry]], [[inductively coupled plasma]], or [[ion selective electrode]]s. -->

Neutral potassium atoms have 19 electrons, one more than the configuration of the [[noble gas]] [[argon]]. Because of its low first [[ionization energy]] of 418.8{{nbsp}}kJ/mol, the potassium atom is much more likely to lose the last electron and acquire a positive charge, although negatively charged [[alkalide]] {{chem2|K−}} ions are not impossible.<ref name="K-">{{cite journal|journal = [[Angewandte Chemie International Edition]]|year = 1979|last = Dye|first=J. L. |title = Compounds of Alkali Metal Anions|volume = 18|issue = 8|pages = 587–598|doi = 10.1002/anie.197905871}}</ref> In contrast, the second ionization energy is very high (3052{{nbsp}}kJ/mol).