(Updated, December 19, 2023, to fix a bad link and make text consistent with the new link.)
If you click here, you'll see a nice animated illustration of how matter can be broken down into smaller and smaller pieces. Going top to bottom in the linked figure, there's ordinary matter (which is depicted as a crystal), which is comprised of molecules, which are made up of atoms, which are composed of sub-atomic particles. The linked website also provides size estimates for the entities at each level of matter. (Actually, the figure depicts one additional set of entities down the smallness scale, called "strings;" string theory is very controversial, so for now we'll stop at sub-atomic particles.)
Two books really gave me an excellent introduction to sub-atomic particles. They are Supersymmetry by Gordon Kane, and The Fabric of the Cosmos by Brian Greene. Also, the Wikipedia, a free online encyclopedia, has proved very helpful to me. I will be drawing extensively from these sources.
Anyone who took high school chemistry should be familiar with molecules and atoms (my recollection of high school physics is that the subject matter is kept at a more macro level, such as a cart rolling down a hill). Molecules have been defined as:
...two or more atoms joined by shared pairs of electrons in a chemical bond. It may consist of atoms of the same chemical element, as with oxygen (O2), or of different elements, as with water (H2O).
Atoms, such as hydrogen atoms or helium atoms, are of course made of protons (which are postively charged), neutrons (which are neutral), and electrons (which are negatively charged). You can click here for a nice visual depiction of an atom.
An electron is considered a "fundamental" particle, in that it cannot be broken down into anything smaller.
Protons and neutrons -- and this is where things get fun -- can be broken down further into things called quarks. The eminent physicist Murray Gell-Mann introduced the term "quark" to the field, having seen it in James Joyce's book Finnegan's Wake. As we'll see in the course of my postings, physicists are quite adept at coming up with eclectic allusions and witty pieces of nomenclature.
Among the different types of quarks, two are referred to as "up" and "down" quarks. A proton consists of two up quarks and a down quark, and a neutron consists of two downs and an up. The composition of protons and neutrons is shown in the following illustration.
Many of you may have heard of particle accelerators and colliders that physicists use for their research, such as the Stanford Linear Accelerator Center (SLAC) or Fermilab, outside Chicago.
Writes Greene:
Over the past hundred years, physicists have prodded, pummeled, and pulverized matter in search of the universe's elementary constituents. And, indeed, they have found that in almost everything anyone has ever encountered, the fundamental ingredients are the electrons and quarks... electrons and two kinds of quarks, up-quarks and down-quarks, that differ in mass and in electrical charge (pp. 345-346).
Kane adds:
...everything we see in the universe, from the smallest cell to flowers to people to stars, is made of three kinds of matter particles... The three matter particles are the familiar electron and two particles similar to the electron called quarks, the up quark and the down quark (p. 20).
If the sub-atomic particles ended with the up and down quarks, the number of matter constituents would be pretty manageable for the lay observer of physics. As I'll discuss next week, however, there are many more particles of matter.