eli5: What’s the half life of an element?


What do people mean when they say “half life of element X”?

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4 Answers

Anonymous 0 Comments

In science, the “half life” of an element is the amount of time it takes for half of the atoms in a sample of the element to break down or decay. This happens because some elements are not stable and eventually turn into other elements.
It’s kind of like a timer that is always running, but it goes very, very slowly. For example, the half life of a certain element might be 1,000 years. That means it would take 1,000 years for half of the atoms in a sample of that element to decay into something else. After another 1,000 years, half of the remaining atoms would decay. And so on.

Anonymous 0 Comments

Radioa time elements decay through natural processes, turning them from one element into another. Sometimes this occurs through spitting out a Hydrogen nucleus (atom?), sometimes spitting out a proton, and sometimes spitting out a high-energy photon. When this happens, that original element is no longer the same. It becomes a different element. The amount of time it takes for this to happen is truly random for an individual atom of an element, but, when measured against a large enough sample size, a pattern of time emerges, such that at the beginning of the measurement you have 1 large unit (say, a kilogram) of the element. After the average half-life has passed, you have half of that large unit of the element remaining, and the rest is something else (e.g. half a kilogram). Alternatively, you could start with 1 kilogram of the element and start a clock. When you have half as much of the element, the average time has elapsed, thus giving you the half-life of the element. For stable, but still radioactive elements, the half-life can be measured un millions of years or more. For less stables elements (like C14), thousands of years. And for extremely unstable elements (like what used to be called Unilhexium), fractions of a microsecond.

Anonymous 0 Comments

A spoonful of material will have some [10^20 atoms](https://en.wikipedia.org/wiki/Avogadro_constant) in it.

And any single atom may react or decay or *do something* quite randomly, could be within the next second or it could be 2000 years from now. Processes at the atomic level are very very random, as far as when they happen exactly.

So the best way to specify how a material will behave (with all its 10^20 atoms) is to do a statistical average of how long it’ll take all those atoms to decay. Between 1 second and 2000 years, it’ll probably take about 50 years for a good chunk of those atoms to decay, with only a few ‘stubborn’ ones lasting the full 2000 years.

So you say that “the half life” = 50 years. It takes about 50 years for HALF the atoms in a spoonful of material to go boom.

This has the advantage of being independent of “size”. It would also take 50 years for half the atoms in the Earth or the Sun to go boom. The “half life” applies to the element, independently of the actual quantity of it.

Anonymous 0 Comments

Half life is the time taken for half of the radioactive nuclei to decay. Radioactive atoms decay due to their unstable nuclei and create another element.

Because of Quantum Physics, we can’t determine with certainty when one atom decays, however, with a large enough sample, we can estimate the amount of time taken for half of an elemental isotope becomes a different element.