Matter is made of atoms. This fact is familiar to everyone. But the basic structure and behavior of atoms is so fundamental to understanding modern physics, that I want to take some time to review it. It’s so important, in fact, that Richard Feynman began his famous lecture series by saying:
If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms—little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another.
The atomic hypothesis–that matter cannot be infinitely divided–was first proposed around 2500 years ago, but at that time it was pure speculation, and for most of those 2500 years, it wasn’t a very popular idea. Our modern understanding of the structure of atoms was mostly developed around the beginning of the 20th century, with some parts of the theory developed as recently as the 1960s. If you want to know more about this history, the Wikipedia article on atomic theory is a good starting point1. For now, I want to focus on our modern picture of the atom.
Atom is actually a misnomer. It comes from the Greek for “indivisible,” but atoms are actually made of smaller particles: protons, neutrons and electrons. One might naturally ask, “Are these particles made of even smaller particles?” For protons and neutrons the answer is “Yes!” They are made of quarks and gluons. This is the part of the atomic model that was developed in the 1960s, and physicists are still studying the dynamics of how, exactly, quarks and gluons interact within protons and neutrons. Electrons, on the other hand, are elementary. As far as we can tell, they are not made of any smaller particles. (The same is true for quarks and gluons.)
The atomic model most people are familiar with goes something like this: protons and neutrons are tightly bound together in the nucleus of an atom, with electrons orbiting the nucleus in much the same way that planets orbit the Sun. This is called the Bohr model, and it’s about a hundred years old. It is accurate in a lot of ways. Protons and neutrons are, indeed, bound together in the nucleus (by the strong nuclear force, or the strong force for short), and electrons surround the nucleus in orbitals. Electrons are bound in these orbitals by the attraction between the positively charged protons and the negatively charged electrons (neutrons have no electric charge).
However, these electron orbitals are basically nothing like planetary orbits. The Bohr model was only around for about a decade before physicists (including Niels Bohr himself) began to realize that it wasn’t quite right. But the Bohr model still persists as the popular image, (just try doing a Google image search for “atom”) probably because the more accurate model is a bit harder to visualize and explain.
The old Bohr model and the not quite as old electron cloud model. This image was shamelessly swiped from this website.
The modern picture is called the electron cloud model, although I prefer to think of it as the “electron bubble model.” Particles are usually pictured as little balls. This picture is almost, but not quite2, completely wrong. Instead, a free electron is “shaped like” a point (as far as we’ve been able to observe. They may actually be shaped like little strings, or something else.) But when it becomes bound to an atomic nucleus, the electron spreads out and become “shaped like” a bubble surrounding the nucleus3. I prefer the term “bubble” to the more commonly used “cloud” for two reasons. First, it makes it more clear that a single electron4 forms the entire bubble (rather than being a “gas” of a bunch of electrons). Second, the shapes of electron orbitals are very closely related to the ways that a sphere (or bubble) can vibrate.
The first several spherical harmonics. These illustrate both some of the vibrational modes of a bubble, and several of the shapes of electron orbitals.
The electron cloud is much, much bigger than the nucleus; about 100,000 times bigger. To get a sense of this scale, take an orange and go stand at the center of the New Circle Road loop around Lexington, KY5. (It turns out this is roughly at the Chemistry-Physics building on UK campus.) If an (average sized) atomic nucleus were the size of the orange, then an atomic orbital would be about the size of the New Circle Road loop.
How big are atoms themselves? Again let’s look at our orange. If we blew up the orange until it’s the size of the Earth (about 8,000 miles in diameter), then the atoms inside it would be about an inch in diameter. Our reference orange is made up of about 1025 atoms. In long form that’s 10,000,000,000,000,000,000,000,000 atoms or ten tera-tera-atoms.
So atoms are tiny, have a simple but bizarre structure, and make up (nearly) everything. Of course, if atoms make up all the matter we see, feel or otherwise interact with, there must be many different types of atoms. As you may already know (or at least suspect), this has something to do with the number of protons, neutrons and electrons in an atom. But let’s take a break for now. We will talk about that next time.
1I’d love to recommend a good book on the history of atomic theory. Unfortunately, I learned most of it from classes and textbooks, so I don’t know any good books about it (although I’m sure they’re out there).
2For composite particles, like protons, neutrons or entire atoms, the “little ball” picture is still somewhat reasonable.
3These point and bubble shapes are “fuzzed out” due to the uncertainty principle. This is where the term “electron cloud” comes from. And for any physicists reading this: yes, I’m hand-waving the whole topic of probability distributions for now.
4Because electrons have a property called spin, up to two electrons can occupy the same orbital (the same bubble shape) at a time. Spin is an entire topic in itself that I won’t get into right now.
5If you’re not in Lexington, any geographical feature with a diameter of about 5-6 miles will work.