Half Lifes

[cardnals100]

So in my high school chemistry class we're learning about half lives. And my teacher made the comment that elements could never reach zero, at first I was questioning how that worked but eventually gave up,so then I wondered about how any elements that go through half lives can be mined,found,etc if they're constantly going through half lives?

Basically what I'm wondering is do elements with half lives always have them? Or are activated at some point when they become radioactive?

[Dolgorukiy]

The overwhelming majority of elements and their isotopes have a measurable half life period, the length of said period depends on the stability of the nucleus, and ranges from microseconds (the shortest discovered to date) to ~10^24 years (the longest found yet). Uranium 238 for instance has a half life period of 4.28x10^9 years which is about the age of our solar system, meaning that about half of the original amount of U238 remains. The half life of U235 which is less stable (and therefore more readily fissile) is only ~7x10^8 yrs, meaning that it still occurs naturally but in rather small amounts as it underwent more half-life cycles (and in no way am I implying that U238 & U235 were originally in the same amounts).

Some elements and/or isotopes have half life cycles so small that they are not naturally occurring for example: Pu-239 - 2.4x10^5 yrs. meaning that its natural occurrence anywhere but near recent supernovae stars is negligible.

[Elfdude]

A half life simply means that by X time half of the items you have will probably have decayed. All atoms and materials have a half life because no bond or atom is permanent when we talk about very very long time periods. Radioactive elements have a short half life. They are usually created by supernovae and permeate the universe at different concentrations. Half life is defined in terms of probability. For example if we have one atom and it's half life is 1 second, there's a 50/50 chance that after one second it will decay. In the next second there's a 50/50 chance it will decay. This single atom (through pure chance) could last days or could've decayed at the first chance. With multiple atoms thanks to the law of large numbers (similar to multiple coin-tosses) we can judge what the group will do via this probability so we apply it.

[cardnals100]

Thanks

+rep to both of you!

[Plant]

So in my high school chemistry class we're learning about half lives. And my teacher made the comment that elements could never reach zero, at first I was questioning how that worked but eventually gave up,so then I wondered about how any elements that go through half lives can be mined,found,etc if they're constantly going through half lives?

Basically what I'm wondering is do elements with half lives always have them? Or are activated at some point when they become radioactive?

She means thatwhen you answer the question in an exam, they can never reach zero. Statistically, or using probability modelling, not all of the total elements can undergo decay, as the formula used never reaches zero (no, not even if you input infinity) but in reality, it can occur. Think of it as the difference between rolling a bunch of dice and taking away a die if it rolls a 6. In reality, you will eventually remove them all. In a probability graph you will draw an exponential decay curve, which implies that there will always be a fraction of a die left.

[Simetrical]

The reason these isotopes still occur in significant quantities is because although they decay, they're also regenerated by various processes. For instance, carbon-14 has a half-life of about 6,000 years, decaying to nitrogen-14 (which is stable). But nitrogen-14 in the atmosphere gets hit by neutrons created by cosmic rays, which turns it back into carbon-14. So it goes both ways. Neither effect is much faster than the other, so we wind up with a small amount of carbon-14 all the time.

In other cases, isotopes decay much faster than they're created, and so in fact you do get practically none of the isotope occurring in nature.