Radioactive decay

Kim’s chair seems quite wobbly and unstable! And now it broke! Are you ok, Kim? Unstable objects are bound to break sooner or later. But did you know that unstable atoms can break down too?

Take this carbon atom. At its core, it has a nucleus consisting of six protons and six neutrons. The nuclear forces that hold the protons and neutrons together are well-balanced, so the atom's nucleus remains unchanged. This is a stable nucleus. A stable nucleus is like a very sturdy chair — it won’t just break!

Now take a look at this atom: it has six protons, so it’s a carbon atom too! But instead of six, there are eight neutrons in the nucleus — so fourteen particles altogether. This is a variant — another isotope — of carbon, called carbon-14. The increased number of neutrons causes an imbalance of forces inside the nucleus — the nucleus is unstable. To achieve a balanced, stable state, the atom needs to get rid of excess energy — so it breaks down spontaneously.

The nucleus of this unstable parent atom changes, producing a new daughter atom. We call this spontaneous breakdown of the atomic nucleus radioactive decay. Most commonly radioactive decay happens in one of three ways. The first type of decay usually occurs in atoms of heavy elements, that have many protons and neutrons. In this case, the parent atom releases a particle made of two protons and two neutrons — an alpha particle.

The resulting daughter atom has two protons fewer and two neutrons fewer than the parent atom. It is an atom of a different element! For example, when a uranium atom undergoes this type of decay, it releases an alpha particle and turns into thorium, which has two protons less! This kind of radioactive decay is called alpha decay. Another type of radioactive decay happens in atoms that have too many or too few neutrons compared to protons.

In this case, one of the atom’s neutrons can suddenly change into a proton, or a proton can change into a neutron. At the same time, the parent atom gives off a positively or negatively charged beta particle. The number of protons in the nucleus changes, so the daughter atom is again an atom of a different element! Carbon-14 that we talked about earlier is an example of an isotope that undergoes this type of decay. When a carbon-14 atom decays, one of its neutrons changes into a proton, and the nucleus releases a beta particle in the form of an electron.

The daughter atom has seven protons instead of six — it is a nitrogen atom! This type of radioactive decay is called a beta decay. The third type of decay usually occurs just after an alpha or a beta decay, when there is excess energy left in the nucleus. To achieve stability, the atom releases a concentrated burst of energy called a gamma ray. The daughter atom is the same element as the parent atom, but it has less energy in the nucleus.

This is gamma decay. Sometimes, even after a gamma decay, a daughter atom is still unstable. In that case, the radioactive decay continues. The radioactive daughter atom decays and produces a new daughter atom. This goes on and on until the nucleus is finally stable.

This sequence of radioactive decays is called a decay chain or decay series. Luckily for Kim, a chair is not radioactive — it can only break once, so Kim won’t keep falling!