Radioactive decay: discovery, process and causes

Radioactive decay is the strange and almost mystical ability of one element to naturally spontaneously transform into another. However, these elements tend to emit lethal forms of radiation in the electromagnetic spectrum.

Radioactive decay is most often associated with nuclear disasters, such as in Chernobyl. (will open in a new tab), where, as a result of harmful radioactive decay in the vicinity, the creation of an exclusion zone was required. According to the US Environmental Protection Agency, radioactive decay occurs in atoms that become unbalanced, called radionuclides. (will open in a new tab). These radionuclides follow their own unique decay path, changing into different elements until they reach a stable state.

Discovery of radioactivity

In the late 1800s, scientists discovered X-rays, a high-energy form of radiation, and wondered if other kinds of “rays” were reflected. At the same time, scientists were just beginning to explore the nature of phosphorescence, the ability of some materials to glow under the influence of X-rays.

The French scientist Henri Becquerel experimented with various phosphorescent materials, including uranium. He soon discovered that all phosphorescent materials did not leave an image on a photographic plate if he covered the plate, but uranium did leave an image. This meant that uranium generated its own radiation, different from phosphorescence. According to the fashion of that time, this type of radiation began to be called Becquerel rays.

Related: In search of one of the most elusive particle decays in the universe

Other scientists have taken a trail from there, finding that uranium was indeed quite strange. According to, in the process of emitting radiation, it transmuted into other elements – something no one has ever seen before. (will open in a new tab). They found that other elements are sometimes capable of the same trick. Pierre and Marie Curie discovered radium and polonium and introduced the term “radioactivity”, which quickly won out over “Becquerel rays”.

Black and white photograph of French scientist Henri Becquerel

Henri Becquerel accidentally discovered spontaneous radioactivity. (Image credit: Getty Images)

Types of radioactive decay

Illustration of radioactive decay at the molecular level

Radioactive decay depicted at the molecular level (Image credit: Getty Images)

Curie and other scientists discovered that radioactive elements emit three types of radiation when they undergo this transmutation process.

Alpha radiation can be easily blocked with just a piece of paper.

Beta radiation requires something more massive, like a metal sheet.

It takes a whole block of lead to stop gamma radiation, according to the US Environmental Protection Agency. (will open in a new tab).

What causes radioactive decay?

Only when physicists had a quantum model of the atom were they able to understand what happens to radioactive decay. They found that the atomic nucleus is actually a loose, sticky sack of protons and neutrons bouncing off each other. There was a lot of complex physics going on inside that sticky sack.

First, there is the strong nuclear force responsible for the formation of triples of quarks into neutrons and protons. However, some of this strong force “leaks” from neutrons and protons, and it is this residual force that is responsible for the formation of atomic nuclei.

Second, the protons in the nucleus are positively charged, and like-minded charges absolutely hate being around each other. According to, protons are constantly trying to bounce off each other, but they are usually held back by the bonding power of the strong nuclear force. (will open in a new tab). Sometimes there is also a weak nuclear force capable of converting a proton into a neutron and vice versa.

Most atomic nuclei are stable most of the time; the balance of power within them holds them together. But sometimes things can become imbalanced.

For example, if there are too many protons, the electrostatic repulsion of those protons will destabilize the nucleus. However, if there are too many neutrons, the strong force is not always able to hold everything together and prevent scattering of neutrons.

If the atomic nucleus is not in the lowest possible energy state, radioactive decay can occur as the nucleus shuffles and reorganizes itself to find a new stable situation. Different ways of rearranging the nucleus lead to different types of radiation.

The alpha radiation is actually an entire helium nucleus (two protons and two neutrons) that has broken off from larger elements.

Beta radiation is an electron emitted during the transformation of a proton into a neutron.

According to the US Nuclear Regulatory Commission, gamma radiation is the emission of a high-energy photon. (will open in a new tab).

Is radioactive decay predictable?

Radioactive decay happens quite by accident. If you took one atom, you could never predict exactly when that atom would decay. This is due to the fact that even very unstable nuclei are connected to each other. There is still a powerful force “bag” that holds all the sloshing protons and neutrons inside.

An atom might prefer a different combination of protons and neutrons, but it has to get there somehow. The only way to get there is to remove parts of itself or change one of its components, which requires a bit of energy.

The atom will be in a lower energy state, but it needs a bit of spending to get there.

Graphic representation of one atom

Protons and neutrons in an atom are usually held together by strong forces. (Image credit: Getty Images)

Is radioactive decay a chemical process?

In classical physics, radioactive decay could never occur because it is impossible to expend energy that does not exist. Quantum mechanics allows this, but does it randomly. From time to time, a slightly unstable atomic nucleus spontaneously decays into something else.

While we can never know when an individual atom will decay, we can build statistics on how an entire population of atoms will decay. This is the idea of ​​a half-life.

For example, one piece of uranium contains an almost innumerable number of atoms. Every second you never know which atom will decay, but you can be quite sure that some atom somewhere in the lump will decay. This means that the half-life tells you how long it will take for half a mass of radioactive material to decay, according to HyperPhysics. (will open in a new tab).

Additional Resources

Marie Curie and radioactivity book. (will open in a new tab) This is an excellent illustrated resource for young readers to introduce them to the concept of radioactivity and the pioneering work of Marie Curie.

For older readers, try Radioactivity: The Story of an Enigmatic Science. (will open in a new tab) Marjorie S. Malley.

In this video, Steve Weatherall gives a great Ted-ED presentation on radioactivity. (will open in a new tab).

Learn more by listening to the Ask an Astronaut podcast, available on iTunes. (will open in a new tab) and (will open in a new tab). Ask your question on Twitter using the hashtag #AskASpaceman or by following Paul @PaulMattSutter. (will open in a new tab) and (will open in a new tab).


US Nuclear Regulatory Commission

Back to top button

Adblock Detected

Please consider supporting us by disabling your ad blocker.