My chemistry and atomic knowledge is pretty basic - I now enough to do my job as a paramedic, but not much more…
What does the term half-life refer to? Is it the measure of the time needed for the radioactivity level to reduce to half that of the original?
What happens to a radioactive element when its full-life (as opposed to its half-life) lapses? Does it completely dematerialize, or does it become another element?
ps I had a quick look in the archive, but I couldn’t find anything that matched my question.
Well when its half life elapses, its radioactivity reduces to half its original level. After another half life, it’s reduced to a quarter of its original. Repeat ad infinitum;)
Of course not all radioactive elements decay to a stable isotope of the same element.
However, I still wonder about what the end result is. Radioactivity is essentially the gradual loss of subatomic particles: a proton here, a neutron there. Hence there is a decay of the original substance. The way I figure it, if a substance is losing the essential bits that give it the properties that it has, surely it has to change to become a different substance, or completely decay to nothing.
Be careful here. It’s not really accurate to say “the radioactivity decreases to half the original level.” What’s more accurate is to say that the half life is the time it takes for one half of the material to change into another element via radioactive decay. In fact, the new element is even more radioactive in some cases.
All radioactivity is is atoms flinging particles away because the atomic structure is not stable. An atom’s chemical properties are a result of the number of electrons it has, and the number of electrons, in the element’s stable state, matches the number of protons which, with the neutrons, make up most of the atom’s mass. If an atom kicks out some protons, the element changes. Most of the time it becomes more stable, and thus less radioactive, but sometimes it can decay into a less stable element.
Plutonium 239 for instance, has a half-life of 24,000 years. If you start with a piece of mass X of pure Pu239, after 24,000 years of alpha emmision you’ll have a piece composed of .5X Pu239, and .5X of Uranium 235, which itself has a halflife is 7.04 x 10[sup]8[/sup] years!
As for the eventual result, it’s a little odd because the mathematical model predicts that you will just keep dividing the amount of the original element in half, which reaches 0 in the infinite limit. Realistically, there are a finite number of atoms, and you can’t have half an atom decay (though that’s a moot point because concentration falls below detectable and/or meaningful amounts long before single atoms become an issue).
It decays down to something that is stable that has a lower molecular mass (number of protons, neutrons and electrons). Take uranium with alpha emission at the start (thats the expulsion of a Helium nucleus)
238U 234Th + 4He
then
234Th 234Pa + e (beta emission)
this continues (using steps like above) till you get to the nice and stable 207Pb
In practical terms, after 5 half-lives, you can pretty much discount the original element ([sup]1[/sup]/[sub]32[/sub] remaining), although you’ll still have to deal with the decay products. Some radioactive elements are far more dangerous as heavy metal poisons than they are as radiation sources.