OK, maybe I’m missing something here, but why should there be any uranium left in the world? As I understand it, all uranium should decay at the same rate. Since we do not “make” uranium, then I must presume all the uranium on earth was here since day 1.
If that is true, and knowing that Radon is a bi-product from the decay of uranium…then why hasn’t ALL the uranium decayed by now?
I must be missing something here…any WAGs?
“They’re coming to take me away ha-ha, ho-ho, hee-hee, to the funny farm where life is beautiful all the time… :)” - Napoleon IV
U-238 has a half life of 4,510,000,000 years. U-235 has a half life 713,000,000 years. So the stuff doesn’t exactly fizzle out quick. One of the ways we date the Earth is to measure the ratio of uranium to lead in various rocks.
Radioactive materials do not decay linearly. Instead, they decay according to their “half-life” which, put simply, is the period of time in which half of the atoms in a given sample will have decayed into something else.
As an example, suppose I have a radioactive substance with a half-life of one year and I start with a kilogram of it. After one year, half of it will have decayed and I will still have half a kilogram or 500 grams. After another year I have 250 grams and the year after that 125 grams and so on. As you can see, while the amount of material I have keeps getting smaller it does so at a slower and slower rate. After hundreds of years I will still have a fraction of a gram of material.
Statistically what is happening is that each atom of a radioactive substance has a percentage chance of decaying in a given amount of time. The half-life of the substance is the amount of time in which each atom has had a 50% chance of decaying.
Another way of looking at it is to assume that you have a very large number of coins. You flip all of the coins and get rid of those that come up heads. After one flip you should have half the number of coins you started with. Flip again and throw out the heads and you once again have half the number of coins you started with before that throw.
Substitute atoms for coins and give yourself a few billions of them and you can see how you can still have radioactive materials after billions of years.
“Sometimes I think the web is just a big plot to keep people like me away from normal society.” — Dilbert
There are elements that have a half-life so short that none of it exists in nature. Mendelevium 256, for example, only has a half-life of 30 minutes. I don’t believe these types of elements have ever been found outside a lab.
Not quite - if you’re talking hypothetically about a substance around the same atomic weight as Uranium, then a mole of it weighs about a 250g, so there are about four moles in a kg. Four moles is about 2.4E24 atoms. After 81 years (half-lives), you’d expect to have only one undecayed atom left. Another year and you probably wouldn’t have any.
Yes, I think uranium is one of the stablest of the radioactive elements. Are there any radioactives with longer half-lives? I was thinking maybe radon and maybe that one potassium isotope.
I remember in school we learned that there are only a few grams of francium in the crust of the earth. I don’t know whether this was because most of it is in the mantle and/or core, or because the rest of it just decayed. Too bad, since it should be one of the most reactive alkali metals (at least given its position on the periodic table).
Carbon-14 also has a nice long half-life. I think tritium does as well.
To elaborate on what Ursa Major said, a lot of what are considered artificial elements probably existed at one time, given the huge variety of stuff that can be created by solar events, but it doesn’t last long enough for any of it to be left. I remember reading about the creation of element 109 (they’ve probably named it since then), which had such a short half-life they needed special instruments to get a glimpse of its properties.
Tritium has a half-life of only 12 years. This is a concern to the nuclear weapons people since tritium is a vital component of fusion bombs and there are no tritium manufacturing facilities currently in operation.
Carbon-14 has a half-life of 5,770 years. Not long by geological standards but long relative to recorded human history. There is a lot of C-14 around because it is created continuously in the atmosphere by cosmic rays, IIRC. Biological tissues absorb C-14 while living, but after death the ratio of C-14 to C-12 declines since the tissues are no longer interacting with the atmosphere.
Other long-lived isotopes: Pu-244 = 76 million years; Np-237 = 2.2 million years; several isotopes of uranium, notably U-238 =
4.5 billion years and U-235 = 700 million years; Th-232 14 billion years.
Radon and thorium occur in nature but are not “natural” in the sense that they are by-products of the decay of uranium.
My source is the CRC Handbook of Chemistry and Physics but it’s the 1967-68 edition so it doesn’t go past element 103. I’m going to insist my cubicle mates update their libraries.
“I’ll tell him but I don’t think he’ll be very keen. He’s already got one, you see!”
Ah well, that’s what I get for an off the cuff answer. I admit I hadn’t run the numbers. Still, my general point about decay is valid even if I misstated my example a bit.
“Sometimes I think the web is just a big plot to keep people like me away from normal society.” — Dilbert
Just to submit some information, but Element 110, or Uun, only has a half-life of 270ms, and it just gets worse as you go up that end of the periodic table…