Question: Why are materials radioactive

Luke Humphrey answered on 7 Apr 2026:

Building on Hamish’s excellent answer, you might follow up by asking where these unstable atoms come from. Since we know that radioactive materials decay into stable ones given enough time, wouldn’t we expect them to all be stable by now? What’s making them radioactive?

Atoms won’t just randomly becomes radioactive, they need to get that energy from somewhere. To my understanding there are a few ways:

1. Primordial radioactive material

When the Earth was formed, it was formed from the matter floating around in this area of space. All this material was the leftovers scattered by the previous generation of stars when they reached the end of their lifetimes. Maybe they went supernova and threw the material all over space, for example.

That material was attracted together by gravity, the attractive force which pulls all masses towards all other masses. (On Earth, we think of gravity as always pulling “down” but that’s just because we live on a planet, so that’s where most of the local mass is!)

Anyway, most of this matter (like >99%) became the sun and the remaining <1% or so became all the planets, so everything on Earth is just part of that same "primordial" mass. Some of that material was already radioactive, because the processes in dying stars can be very violent and forceful and that can push material into these unstable (radioactive) atomic arrangements.

Some radioactive decays are extremely slow, so it's not unreasonable that a lot of it is still radioactive today even after so much time has passed. Also, radioactive decay doesn't always leave behind a stable isotope, there's often a long chain of decays before they reach something stable.

So in other words, it was like that when we got here!

2. Spontaneous radioactive material

Some radioactive material is being made every day by other processes, since the whole Earth is constantly being bombarded by cosmic radiation. The atmosphere ends up absorbing a lot of this stuff before it reaches the surface, but a particle "absorbing" radioactive energy often means becoming radioactive itself!

It's a bit like "hot potato" where radioactive atoms decay into stable ones, but the energy they released gets absorbed by another atom and that one becomes radioactive for a while.

Entropy ensures that this game of hot potato will eventually end, as physics doesn't allow "perpetual motion", but the heat death of the universe is a long, long way away (long after the lifetime of the Sun & Earth) so it's not something humans are likely to see.

In other words, the radiation coming from space and the radioactive material we already have here on Earth ends up making more radioactive material as it decays.

3. We make it ourselves

Given that radioactive material is made spontaneously by nature, we can also choose to expose material to the right kind of radiation to turn it into the form we want for whatever purpose. For example, Molybdenum-99 is made intentionally for use in medicine to image people's internal organs for diagnostics. It decays very quickly, so we're constantly making it in nuclear reactors rather than using any natural supply.

Besides that, radioactive waste is also created as a by-product from nuclear reactors. This is material we need to store safely to avoid it from giving people radiation poisoning while we wait for it to decay into inert form.

Radioactive materials related to nuclear weapons are, as you might imagine, also strictly controlled. I think there's a lot of paperwork required when moving around less than one gram of such materials.