What are the fundamental particles that make up a nucleus?

Asked by: James Bortner


Tough question there... This is one of the fundamental problems of physics (well, nuclear/particle physics): To find out what matter really consists of.

Atoms, which are considered to be the building blocks of "ordinary" matter, are made up of electrons, a lot of "empty space", and nuclei. We still believe electrons are fundamental particles. "Empty space" is may be very interesting otherwise (since it is anything but empty space) but we'll ignore that in this case. The remaining part is the nucleus, and that's where we run into a lot of trouble.

Any typical textbook will say that a nucleus consists of neutrons and protons. Neutrons and protons are particles of about equal mass (the neutron is about half a percent heavier), the proton has an electrical charge of +1e (equal to the charge of the electron but of opposite sign) while the neutron is well, neutral.

Is this the end of the story? No. Because we know that protons and neutrons are not fundamental particles, but actually have internal structure, they are composite particles. But what are they made of? It turns out that they seem to be composed of what we call "quarks". So far we know of six quark flavors: Up, down, strange, charm, top, and bottom. However, only two of these quarks (up and down) are stable, and will not decay.

The "up" quark has a charge of (2/3)e, while the "down" quark has a charge of (-1/3)e. The proton seems to be made of two up quarks and a down quark (uud), while the neutron seems to be made of one up quark and two down quarks (udd).

So, you may think the nucleus is made up of up and down quarks. But that is not true either. There are also "gluons", which are the force mediating particles that hold quarks together and are ultimately also responsible for the nuclear force that binds the whole nucleus together.

Is that all? No, still not. Apart from the quarks that constitute the nucleons (i.e. neutrons and protons) (these are called "valence quarks") there also exists a "sea of quarks", which continually pop into and out of existence due to quantum fluctuations.

So, a nucleus is quite a compilcated thing. But you can say, as far as fundamental particles go, the nucleus is made up of quarks and gluons...
Answered by: Yasar Safkan, Ph.D., Software Engineer, Noktalar A.S., Istanbul, Turkey