Why exactly do radioactive elements decay in half every set amount of time? What is happening on an atomic scale?

Question

Why exactly do radioactive elements decay in half every set amount of time? What is happening on an atomic scale?

In: Physics

Answer 1

To the best of my knowledge we don’t know the full answer to that question. What we do know is this:

* Whether or not an atom of a radioactive element decays at any given moment is essentially random.
* Different elements are more or less susceptible to decay (i.e., some elements have a higher probability of decaying at any given moment than other elements have).

Imagine you have 100 atoms of a radioactive element. Every second you roll one 6-sided die for each atom, and on a roll of 1 that atoms decays. When you pick up your big handful of dice after the first second and roll them you can reasonably expect that roughly 1/6th of all rolls will be 1s, leaving you with around 83 undecayed atoms. For the next second, you pick up your handful of dice and roll them and can reasonable expect that 1/6th of all rolls will be 1, leaving you with around 69 undecayed atoms. Repeat again and you get 58, and then 48, and then 40, and so on. The half-life would be somewhere between the third and fourth seconds, so this element would have a half-life of about 3.5ish seconds.

Notice that the number of atoms that decay in each iteration gets smaller each time (17, 14, 11, 10, 8, …) because you’re losing a consistent *percentage* of atoms, even though every atom has a consistent probability of decaying during any given second: each second it has a 1-in-6 chance of decaying. It’s kind of a weird property of probability that at large enough scales random events become very predictable, and half-life is an example of that.

The thing we *don’t* know, as far as I’m aware (and someone please correct me if I’m wrong here), is what governs whether or not an atom decays at any given moment. It’s probably not *literally random*, it’s probably caused by something in the atom’s structure or the fabric of spacetime or… who knows what. But there’s a lot about how the universe works at that scale that we’re still trying to puzzle out.