> Isotopes of a given element have the same number of protons (92 for uranium), but different numbers of neutrons (143 for U-235 and 146 for U-238), meaning they behave the same chemically but differ in mass or radiation emissions.<p>A fun but off-topic note: "behave the same chemically" is only approximately true. For heavy atoms like the ones discussed in the article, it's basically true. But for hydrogen, adding one neutron <i>doubles</i> its mass and you can get real effects on chemical reaction rates.<p><a href="https://en.wikipedia.org/wiki/Kinetic_isotope_effect" rel="nofollow">https://en.wikipedia.org/wiki/Kinetic_isotope_effect</a><p>And of course, an obligatory "In The Pipeline" link on how it's used in drug discovery: <a href="https://www.science.org/content/blog-post/isotopes-get-your-revivifying-isotopes" rel="nofollow">https://www.science.org/content/blog-post/isotopes-get-your-...</a>