Editing Nobelium (section)

===Isotopes===
{{Main|Isotopes of nobelium}}

Fourteen isotopes of nobelium are known, with [[mass number]]s 248–260 and 262; all are radioactive.{{NUBASE2020|ref}} Additionally, [[nuclear isomer]]s are known for mass numbers 250, 251, 253, and 254.<ref name="unc">{{Cite web | url=http://www.nucleonica.net/unc.aspx | title=Nucleonica :: Web driven nuclear science}}</ref><ref name="NUBASE2003" /> Of these, the longest-lived isotope is <sup>259</sup>No with a half-life of 58&nbsp;minutes, and the longest-lived isomer is <sup>251m</sup>No with a half-life of 1.7&nbsp;seconds.<ref name="unc" /><ref name="NUBASE2003">{{NUBASE 2003}}</ref> However, the still undiscovered isotope <sup>261</sup>No is predicted to have a still longer half-life of 3&nbsp;hours.{{NUBASE2020|ref}} Additionally, the shorter-lived <sup>255</sup>No (half-life 3.1&nbsp;minutes) is more often used in chemical experimentation because it can be produced in larger quantities from irradiation of [[californium-249]] with [[carbon-12]] ions.<ref name="Silva16378" /> After <sup>259</sup>No and <sup>255</sup>No, the next most stable nobelium isotopes are <sup>253</sup>No (half-life 1.62&nbsp;minutes), <sup>254</sup>No (51&nbsp;[[second]]s), <sup>257</sup>No (25&nbsp;seconds), <sup>256</sup>No (2.91&nbsp;seconds), and <sup>252</sup>No (2.57&nbsp;seconds).<ref name="Silva16378" /><ref name="unc" /><ref name="NUBASE2003" /> All of the remaining nobelium isotopes have half-lives that are less than a second, and the shortest-lived known nobelium isotope (<sup>248</sup>No) has a half-life of less than 2&nbsp;[[microsecond]]s.{{NUBASE2020|ref}} The isotope <sup>254</sup>No is especially interesting theoretically as it is in the middle of a series of [[prolate]] nuclei from <sup>231</sup>[[protactinium|Pa]] to <sup>279</sup>[[roentgenium|Rg]], and the formation of its nuclear isomers (of which two are known) is controlled by [[nuclear shell model|proton orbitals]] such as 2f<sub>5/2</sub> which come just above the spherical proton shell; it can be synthesized in the reaction of <sup>208</sup>Pb with <sup>48</sup>Ca.<ref name="Kratz">{{cite conference |last1=Kratz |first1=Jens Volker |date=5 September 2011 |title=The Impact of Superheavy Elements on the Chemical and Physical Sciences |url=http://tan11.jinr.ru/pdf/06_Sep/S_1/02_Kratz.pdf |conference=4th International Conference on the Chemistry and Physics of the Transactinide Elements |access-date=27 August 2013 }}</ref>

The half-lives of nobelium isotopes increase smoothly from <sup>250</sup>No to <sup>253</sup>No. However, a dip appears at <sup>254</sup>No, and beyond this the half-lives of [[even and odd atomic nuclei|even-even]] nobelium isotopes drop sharply as [[spontaneous fission]] becomes the dominant decay mode. For example, the half-life of <sup>256</sup>No is almost three seconds, but that of <sup>258</sup>No is only 1.2&nbsp;milliseconds.<ref name="Silva16378">{{harvnb|Silva|2011|pp=1637–8}}</ref><ref name="unc" /><ref name="NUBASE2003" /> This shows that at nobelium, the mutual repulsion of protons poses a limit to the [[island of stability|region of long-lived nuclei]] in the [[actinide]] series.<ref name="Nurmia">{{cite journal |first=Matti |last=Nurmia |date=2003 |title=Nobelium |journal=Chemical and Engineering News |url=http://pubs.acs.org/cen/80th/nobelium.html |volume=81 |issue=36 |page=178 |doi=10.1021/cen-v081n036.p178 }}</ref> The even-odd nobelium isotopes mostly continue to have longer half-lives as their mass numbers increase, with a dip in the trend at <sup>257</sup>No.<ref name="Silva16378" /><ref name="unc" /><ref name="NUBASE2003" />