Ok, correct my science and educate me. As I uunderstand it, all heavier elements (gold, uranium, etc.) were created during the “death” of stars, and the overwhelming percentage of those elements found on earth were present at earth’s formation - about 4 billion years ago. (Am I correct so far?)
It seems to me, then, that these heavier elements must be mostly more than 4 billion years old (probably much more). So why do elements that decay at a much quicker rate still exist naturally on earth?
Thanks in advance for the anticipated enlightenment.
Even though the half-life of an element may be less than 4 billion years, if there was a lot of it to begin with, some will still be present, because only half of it is gone at the end of each half-life period.
More importantly, short-lived elements are constantly being created as decay products from longer-lived elements. Take Radon, for example. It has a very short half-life, but it constantly being created by the decay of Uranium.
Consider an anazlogy: “If X million cubic feet of water exit the Mississippi into the Gulf of Mexico daily, why is there still water in it at St. Louis?” The answer, of course, is that water is constantly entering the system upstream.
Likewise, there are three metastabke* nuclides which are among the heaviest naturally occurring elements: Thorium-232, with a half-life of 10.7 billion years, and Uranium-238 and -235, with 4.5 and 0.7 billion year half-lives respectively. As can be seen, about 80% of Thorium, just about 50% of U-238, and around 1.5% of U-235 remain in eistence from when the Earth was formed. Though none of their decay products much exceeds a million-year half-life, and typically the decay products have half-lives ranging from a few days to a few thousand years, the amount of each of them remains constant (over historical time; declining on a geological time scale) since it is replenished at about the rate it breaks down.
The term metastable has two uses, one for the long-lived isotopes described here, and the other for the “excited” (higher ener4gy) state some breakdown products first decay to, before losing the excess energy as gamma radiation and dropping to the ‘unexcited’ state.
The most extreme example is astatine, the most stable isotope of which has a half-life of just over 8 hours; it is estimated that less than one ounce exists in the Earth’s crust at any one time.
The extreme examples are Francium and Astatine, both of which are estimated to be present on the Earth in a quantity of 30 grams or so at any given moment. The most stable Francium isotope has a half life of 22 minutes, the most stable Astatine isotope 8 hours.
Note that the decay chains for Thorium and Uranium contain passage through isotopes of many elements with half lives measured in seconds or milliseconds. Francium and Astatine just don’t happen to have any isotopes with half lives of even a day, so trying to isolate any of the stuff in nature is pretty much a useless exercise. Astatine can be produced in a reactor should you want some for a few hours.
ETA:
crossed the previous post, and anyway, Francium is slightly more extreme. My source is webelements
Because of their half-lives. Unstable elements have half lives. At the end of the first half life, only half the element has decayed. So there’s half of it left. At the end of the next half-life period, half of it again has decayed, so there’s a quarter left, and so on. These half lives can be in the hundreds of millions of years, so if you start with a very large amount, you’ll still have stuff around today because there have been relatively few iterations.