Not homework, for a final. I have pages of notes on stable isotopes - examples of them, how they are used, the del equation and so on, but no definition. I know what an isotope is (an atom that has more or less neutrons than protons, hence a different atomic mass) and I have examples of stable isotopes ([sup]12[/sup]C, [sup]13[/sup]C), but what makes it stable? In other words, what are stable isotopes? How are they different from regular isotopes? Are regular isotopes unstable? Does it have to do with radioactive breakdown, or what?
Well, the number of electrons in orbitals in an uncharged atom, which is of course equivalent to the number of protons in the nucleus, determines its chemical properties, and hence what element it is. Each atom is comprised of nucleons (protons and neutrons) in the nucleus, and electrons in orbitals around it.
An atom with P protons and N neutrons is termed a nuclide; the different nuclides for a given element are its isotopes. (An atom with only one nuclide has no isotopes in the same way as a single child is not a twin, a bit of linguistic usage that need not concern us further.)
Stable nuclides are ones that do not break down radioactively, by alpha or beta decay or spontaneous fission. Metastable nuclides are ones that occur naturally but do break down over periods of time on the order of half a billion years and up, such as U-235. (The most common isotope of indium is metastable, but has a half-life of a quintillion years, so that it doesn’t even appear radioactive to normal testing.) Unstable or radioactive isotopes, also called radioisotopes, break down in shorter periods, though many do occur in nature, as breakdown products of metastable isotopes or created by irradiation. Examples of natural unstble isotopes are Radium-226, a breakdown product of U-238, and Carbon-14, mostly created by irradiation of Nitrogen-14 in the upper atmosphere.
A “normal” or “natural” isotope is, IIRC, simply one that occurs in nature. whether stable, metastable, or one of the unstable ones with a reason for existence in nature.
Why a nuclide is stable, metastable, or unstable is complex, and requires some interesting discussions of subnuclear mechanics that I’m not up on, but it can be quickly summarized:
The strong nuclear force holds nucleons together, while the electric charge tends to repel things of like charge. This means that two or more positively-charged protons require moderation by uncharged neutrons to coexist without repelling each other strongly enough to overcome the SNF. Helium-3, “tralphium,” is the only stable isotope with more protons than neutrons (except, of course, Hydrogen-1, “protium” which has no neutron); above Neon-20 it takes a greater number of neutrons than there are protons to stabilize a nucleus. (Most of the low-mass elements have a predominant isotope of equal numbers of protons and neutrons, plus another one or two isotopes with “excess” neutrons. Above neon, the common isotopes are always made up of n protons and n+x neutrons.) Bismuth-209 is the heaviest stable nuclide (and the only stable isotope of bismuth); that we have naturally-occuring heavier elements is due to the three metastable nuclides Thorium-232, Uranium-238, and Uranium-235, which break down into the elements between.
Thanks! One more question, and I hope I don’t sound too thick - when you say stable nuclide, is that the same as a stable isotope? I know we discussed nuclides in chemistry (briefly), this stable isotope thing is for a different class and I get confused by terminology very easily.
In one word, yes.
Nuclide is the more general term; “isotope” is what everybody uses for nuclide, but which actually means “one of two or more nuclides of a given element.”