Similar to quarks, which stick together through the exchange of gluons, protons and neutrons stick together through the exchange of mesons. Instead of exchanging colors, they exchange quarks. So a particle that is a proton may send out a meson and be a neutron the next moment. You can see this illustrated in the animation on the right.

The particles created when protons and neutrons stick together through the exchange of mesons are called nuclei (singular: nucleus). Since protons and neutrons are the only particles found inside a nucleus, we also call them nucleons.

In contrary to quarks that can only appear in special numbers since the overall hadron has to be color neutral, the protons and neutrons don't have to worry about special numbers. After all, they are color neutral from the beginning. So you can find nuclei with any number of nucleons. For example, a helium nucleus usually has two protons and two neutrons. A meitnerium nucleus has 109 protons and 159 neutrons.

Even though the nucleons don't have to worry about making a color neutral particle, they have other things to worry about. If the ratio between the protons and neutrons isn't right, the different forces inside the nucleus won't be stable. There will be changes in the nucleus until the forces have stabilised. We call these changes decays.

You can build your own nucleus on the right and check if it's a stable one or not. We'll take a closer look at the different decays later.

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