Why is there uranium?

The earth is 4 billion years old. Uranium has a half life of like a few hundred thousand years. Why is there still enough uranium around for us to make so many reactors and bombs?

Actually, [sup]238[/sup]U has a half-life of 4.5 billion years. So we still have about half of the uranium we started with.

Other isotopes have shorter half-lives (don’t have those stats). But not knowing how much we started with, it’s tought to say how much we should have now.

I remember looking at a chart of these things in 10th grade Chemistry, and something had a half-life of 3 minutes. Does that mean you could just sit there and watch a lump of it waste away?

Unstable elements still exist primarily because they’re still being produced. You start off with a few long-halflife isotopes, such as U-238 or one of the isotopes of thorium (I don’t remember which, alas). Every so often, an atom of one of them decays, almost always to some other radioactive element (it typically takes about 8 separate decays for U-238 to turn into a stable isotope of lead). The important point is that the daughter atom produced very often has a much shorter half-life than the parent atom. Since the amount of U-238 is changing only very slowly, the amount of its daughters is also changing very slowly.

Gunslinger, you could watch it, but I wouldn’t advise it. Low halflife substances decay so quickly by throwing off a Hell of a lot of radiation. Also note that it doesn’t just disappear, it turns into something else, so you could watch it change, but not just waste away.

From http://www.fas.org/nuke/hew/Nwfaq/Nfaq6.html

These are the %abundance and half-lives of naturally occuring Uranium isotopes.
______Mass % Atomic % Half Life
U-234 0.0054% 0.0055% 247 thousand years (alpha emitter)
U-235 0.7110% 0.7202% 710 million years (alpha emitter)
U-238 99.2836% 99.2742% 4.51 billion years (alpha emitter)

As expected the natural abundance follows the length of half life fairly closely.

Well, you wouldn’t see it waste away so much as watch it change chemically and give off a lot of radiation. Really high numbered elements have been created in accelerators and they generally have half lives on the order of miliseconds; they’re just too unstable to exist for very long. (Consequently, you can’t make very much of it.)

For obvious reasons, those elements don’t exist in nature.

Which is also the age of the Earth…interesting coincidence.

Actually, it was just a guess, the age of the earth, not a coincidence. A bunch of guys were sitting around on foutons and bean bag chairs smoking pot and one of them asked “Dude, like, how old’s the planet, man?” And another one said “URANIUM!” And everyone laughed. The next morning (after a few generous hits of LSD) they made the obvious connection (well, obvious when you’re tripping).

U571 only lasted a few weeks at the box office. :wink: