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Q & A: isotopes

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Most recent answer: 10/22/2007
Q:
okay. the question is, why do different atoms of elements have different #’s of particles? shouldnt they all be alike?
- bob
A:
I think you're asking about different isotopes, such as carbon 12 and carbon 13. For a given number of protons (say 6), different numbers of neutrons (say 6 or 7) can bind with the protons to form stable nuclei. So these are really different nuclei. Why then do we call the atoms that they make when they bind electrons atoms of the same element (say carbon again)? The total electrical charge on the nucleus is determined by the number of protons. The number of electrons bound in a neutral atom then just matches the proton number. Most of the chemical behavior is determined by these bound electrons. Only some relatively subtle chemical effects depend on the mass of the nuclei. So we lump together these atoms with similar chemistry but slightly different masses under the name of a single element.

Although the effects of isotope mass on chemistry are usually subtle, they sometimes are quite prominent. That's particularly true for hydrogen and its isotope deuterium, which differ by a factor of two in mass. That's enough to effect chemical reaction rates and equilibira a lot. I believe that if you tried switching to drinking heavy water (with deuterium) it would be enough different to be fatal. On the other hand, some electrical engineers here have found that using deuterium rather than hydrogen in processing silicon chips can dramatically extend the chips' lifetime. The different rates of biochemical reactions for different isotopes leave identifiable isotope concentration changes in biological residues, which can be handy in analyzing deposits.

Mike W.

On the more general question of why there are different kinds of elements: Protons and neutrons stick together with the strong nuclear force. Many different combinations of protons and neutrons can be bound together with this very strong force, but some combinations are more stable than others. If you put too many together, then a nucleus can have more energy being put together than the energy of two smaller nuclei plus a little kinetic energy. The nuclei of atoms like these spontaneously fall apart -- usually into an alpha particle (two protons and two neutrons bound together very stably) and whatever's left. This process may repeat. Known stable nuclei have to be stable with respect to alpha decay -- the stablest have this decay energetically prohibited.

Another kind of nuclear decay is called beta decay, and its cousin, inverse beta decay or inner-shell electron capture. If there are many more neutrons than protons for example, it can be energetically favorable for a neutron to decay into a proton by emitting an electron and a neutrino, and thus fall into a lower energy level (protons and neutrons fill up separate energy levels inside the nucleus. If the neutron ones are filled up to a higher level than the protons, energy can be gotten by this decay process). Protons can turn into neutrons by grabbing an electron and emitting a neutrino. These only happen if the energy works out to allow it.

It turns out that these two processes, alpha decay and beta decay, allow only a subset of the possible combinations of protons and neutrons to form stable nuclei. The stablest nuclei have approximately as many protons as neutrons, and elements with more than about 100 protons are all unstable. Different isotopes of common elements can have different lifetimes and decay to other elements.

Tom

(published on 10/22/2007)

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