Editing Period 6 element (section)

===Caesium===
{{main|Caesium}}
'''Caesium''' or '''cesium'''{{#tag:ref|''Caesium'' is the spelling recommended by the [[International Union of Pure and Applied Chemistry]] (IUPAC).<ref>{{RedBook2005|pages=248–49}}.</ref> The [[American Chemical Society]] (ACS) has used the spelling ''cesium'' since 1921,<ref>{{Cite book|editor1-first = Anne M.|editor1-last = Coghill|editor2-first = Lorrin R.|editor2-last = Garson|year = 2006|title = The ACS Style Guide: Effective Communication of Scientific Information|edition = 3rd|publisher = American Chemical Society|location = Washington, D.C.|isbn = 978-0-8412-3999-9|page = [https://archive.org/details/acsstyleguideeff0000unse/page/127 127]|url = https://archive.org/details/acsstyleguideeff0000unse/page/127}}</ref><ref>{{Cite journal|journal=Pure Appl. Chem.|volume=70|issue=1|last1=Coplen|pages = 237–257|year = 1998|first1=T. B.|url =http://old.iupac.org/reports/1998/7001coplen/history.pdf|last2=Peiser|first2=H. S.|title = History of the recommended atomic-weight values from 1882 to 1997: a comparison of differences from current values to the estimated uncertainties of earlier values|doi = 10.1351/pac199870010237|s2cid=96729044}}</ref> following ''Webster's New International Dictionary''. The element was named after the Latin word ''caesius'', meaning "bluish gray". More spelling explanation at [[American and British English spelling differences#ae and oe|ae/oe vs e]].|group=note}} is the [[chemical element]] with the symbol '''Cs''' and [[atomic number]] 55. It is a soft, silvery-gold [[alkali metal]] with a melting point of 28&nbsp;°C (82&nbsp;°F), which makes it one of only five elemental metals that are liquid at (or near) [[room temperature]].{{#tag:ref|Along with [[rubidium]] (39&nbsp;°C [102&nbsp;°F]), [[francium]] (estimated at 27&nbsp;°C [81&nbsp;°F]), [[mercury (element)|mercury]] (−39&nbsp;°C [−38&nbsp;°F]), and [[gallium]] (30&nbsp;°C [86&nbsp;°F]); bromine is also liquid at room temperature (melting at −7.2&nbsp;°C, 19&nbsp;°F) but it is a [[halogen]], not a metal.<ref>{{Cite web|url=http://www.webelements.com/|publisher=University of Sheffield|access-date=2010-12-01|title=WebElements Periodic Table of the Elements}}</ref>|group=note}} Caesium is an [[alkali metal]] and has physical and chemical properties similar to those of [[rubidium]] and [[potassium]]. The metal is extremely reactive and [[pyrophoricity|pyrophoric]], reacting with water even at−116&nbsp;°C (−177&nbsp;°F). It is the least [[electronegativity|electronegative]] element having a stable isotope, caesium-133. Caesium is mined mostly from [[pollucite]], while the [[Radionuclide|radioisotopes]], especially [[caesium-137]], a [[fission product]], are extracted from waste produced by [[nuclear reactor technology|nuclear reactors]].

Two German chemists, [[Robert Bunsen]] and [[Gustav Kirchhoff]], discovered caesium in 1860 by the newly developed method of [[Atomic emission spectroscopy#Flame emission spectroscopy|flame spectroscopy]]. The first small-scale applications for caesium have been as a "[[getter]]" in [[vacuum tube]]s and in [[Solar cell|photoelectric cells]]. In 1967, a specific frequency from the [[emission spectrum]] of caesium-133 was chosen to be used in the definition of the [[second]] by the [[International System of Units]]. Since then, caesium has been widely used in [[atomic clock]]s.

Since the 1990s, the largest [[#Applications|application of the element]] has been as caesium formate for [[drilling fluid]]s. It has a range of applications in the production of electricity, in electronics, and in chemistry. The radioactive isotope caesium-137 has a [[half-life]] of about 30 years and is used in medical applications, industrial gauges, and hydrology. Although the element is only mildly toxic, it is a hazardous material as a metal and its radioisotopes present a high health risk in case of radioactivity releases.