Editing Rubidium (section)

===Isotopes===
{{Main|Isotopes of rubidium}}
Rubidium in the Earth's crust is composed of two isotopes: the stable <sup>85</sup>Rb (72.2%) and the [[radioactive]] <sup>87</sup>Rb (27.8%).<ref name="Audi">{{NUBASE 2003}}</ref> Natural rubidium is radioactive, with specific activity of about 670 [[Becquerel|Bq]]/g, enough to significantly expose a [[photographic film]] in 110 days.<ref>{{cite journal | last1 = Strong | first1 = W. W. | title = On the Possible Radioactivity of Erbium, Potassium and Rubidium | journal = Physical Review | series = Series I | volume = 29 | issue = 2 | pages = 170–173 | date = 1909 | doi = 10.1103/PhysRevSeriesI.29.170 |bibcode = 1909PhRvI..29..170S | url = https://zenodo.org/record/1545957 }}</ref><ref>{{cite book | url = https://books.google.com/books?id=6khCAQAAIAAJ | pages = 4–25 | title = CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data | isbn = 978-0-8493-0476-7 | author1 = Lide, David R | author2 = Frederikse, H. P. R | date = June 1995| publisher = CRC-Press }}</ref> <!--CRC rubber bible gives 30 to 60 days but I could not find a source in science literature.-->Thirty additional rubidium isotopes have been synthesized with half-lives of less than 3&nbsp;months; most are highly radioactive and have few uses.<ref>{{cite web |url=http://www.nucleonica.net/unc.aspx |title=Universal Nuclide Chart |publisher=nucleonica |url-access=registration|accessdate=2017-01-03 |archive-date=2017-02-19 |archive-url=https://web.archive.org/web/20170219043412/http://www.nucleonica.net/unc.aspx |url-status=live }}</ref>

Rubidium-87 has a [[half-life]] of {{val|48.8|e=9}}&nbsp;years, which is more than three times the [[age of the universe]] of {{val|13.799|0.021|e=9}}&nbsp;years,<ref name="Planck 2015">{{cite journal
 |author=Planck Collaboration
 |year=2016
 |title=Planck 2015 results. XIII. Cosmological parameters (See Table 4 on page 31 of pfd).
 |arxiv=1502.01589
|doi=10.1051/0004-6361/201525830
 |bibcode=2016A&A...594A..13P
 |volume=594
 |journal=Astronomy & Astrophysics
 |page=A13
 |s2cid=119262962
 }}</ref> making it a [[primordial nuclide]]. It readily substitutes for [[potassium]] in [[mineral]]s, and is therefore fairly widespread. Rb has been used extensively in [[rock dating|dating rocks]]; <sup>87</sup>Rb [[beta decay]]s to stable <sup>87</sup>Sr. During [[Fractional crystallization (geology)|fractional crystallization]], Sr tends to concentrate in [[plagioclase]], leaving Rb in the liquid phase. Hence, the Rb/Sr ratio in residual [[magma]] may increase over time, and the progressing [[Igneous differentiation|differentiation]] results in rocks with elevated Rb/Sr ratios. The highest ratios (10 or more) occur in [[pegmatite]]s. If the initial amount of Sr is known or can be extrapolated, then the age can be determined by measurement of the Rb and Sr concentrations and of the <sup>87</sup>Sr/<sup>86</sup>Sr ratio. The dates indicate the true age of the minerals only if the rocks have not been subsequently altered (see [[rubidium–strontium dating]]<!-- the hyphen is correct, it uses Rb/Rb and Sr/Sr ratios -->).<ref>{{Cite book |chapter-url = https://books.google.com/books?id=k90iAnFereYC&pg=PA162 |chapter = Rubidium-Strontium Dating |title = Isotopes in the Earth Sciences |first1 = H.-G. |last1 = Attendorn |first2 = Robert |last2 = Bowen |publisher = Springer |date = 1988 |isbn = 978-0-412-53710-3| pages = 162–165}}</ref><ref>{{Cite book |chapter-url =https://books.google.com/books?id=cYWNAZbPhMYC&pg=PA383 |title = Essentials of geochemistry |first1 =John Victor |last1 =Walther |publisher =Jones & Bartlett Learning|orig-year=1988 | date = 2009 |isbn =978-0-7637-5922-3| chapter =Rubidium-Strontium Systematics| pages = 383–385}}</ref>

[[Rubidium-82]], one of the element's non-natural isotopes, is produced by [[electron capture|electron-capture]] decay of [[strontium-82]] with a half-life of 25.36&nbsp;days. With a half-life of 76 seconds, rubidium-82 decays by positron emission to stable [[krypton-82]].<ref name="Audi" />