What's in an Atom*

Electrons and nuclei

At first glance..., well, not really at first glance, since at first glance you can't even see an atom, because they're so small, but if you could see them.... Hmm. Let's try it again...

Electrons and nuclei

Not long after the discovery of the atom, it was discovered that the atom is made up of two different regions. Now the way this was discovered is the way that most things are learned about microscopic particles. That is, you bombard them with other microscopic particles, and see what happens. This is what Ernest Rutherford was doing early in this century, in an experiment in which gold atoms were bombarded with alpha particles. (Don't worry about what alpha particles are. It's not important here.) He found the results quite remarkable. The experiment used a very thin gold foil as the target, and Rutherford expected most of the alpha particles to go on through, and be detected on the other side. What he found was that a few of the particles, and not nearly as few as he expected, bounced back more or less in the direction from which they came. Rutherford tried to figure out how this could happen, and the best scenario he could come up with was, in fact, the truth. The bulk of an atom is concentrated in a very small region at the center. When an alpha particle hits this region, it bounces back. When it misses, it goes on through.

Now the atom was already known to contain tiny particles called "electrons," which carry a negative electric charge. (We'll get to electric charge in the next page.) Rutherford found that the region at the center, which we now call the "nucleus," carries a positive charge. The electrons aren't in the nucleus, but instead surround it. So the atom is found to be in two parts: a tiny nucleus at the center, which contains a positive charge, and the surrounding region, which is sparsely populated by negatively charged electrons.

Protons and neutrons

Physicists didn't stop here. They found new, more powerful, more exotic ways to bombard atoms, and built better detectors to measure the result. And this gave a much clearer picture of what's inside an atom.

The nucleus is populated by two kinds of particles: positively charged "protons," and electrically neutral particles, called "neutrons." These particles are nearly identical to each other, except in electric charge. Protons and neutrons ("nucleons," as they're referred to collectively) are much heavier than electrons, and that's why most of the mass of an atom is in the nucleus.

Atoms are classified according to the number of protons in their nuclei. For example, any atom with six protons in its nucleus is a carbon atom. Any atom with one proton is a hydrogen atom. 26 protons means iron, 92 protons for uranium, and so on.

The number of neutrons in an atom can vary, but is typically close to the number of protons, for light elements (atoms with small numbers of protons). For example, the most common variety of carbon has six neutrons, in addition to its six protons. 6 protons + 6 neutrons = 12 nucleons, and so this variety is called Carbon-12. Many carbon atoms, however, have 8 neutrons. 6 + 8 = 14, so this type of carbon is Carbon-14, something you may have heard of in the context of radioactive dating, for example. Different varieties of the same element with different numbers of neutrons are called different "isotopes." As an example of a heavier element, lead atoms have 82 protons, and the most common isotope of lead has 126 neutrons. Heavy elements typically have many more neutrons than protons.

The number of electrons in an atom is typically the same as the number of protons, which makes the atom as a whole electrically neutral. (In a carbon atom, for example, the positive charge of six protons is exactly cancelled by the negative charge of six electrons.) Atoms with more electrons than protons, or vice versa, are electrically charged, and are called "ions."

Quarks and gluons

But it doesn't stop here. Well, for electrons, it stops here. Electrons, as far as we can tell, are fundamental. They're not made up of anything smaller. But for protons and neutrons, it doesn't stop here. Protons and neutrons are made up of smaller particles of two types: quarks and gluons. Quarks are tiny, charged particles that come in six varieties: up, down, charm, strange, top, and bottom. (To a particle physicist, these names are actually rather descriptive. The word "quark," on the other hand, is completely whimsical.) Only the up and down varieties are important in protons and neutrons. The other quarks can make up particles similar to protons and neutrons, but usually heavier. These particles decay almost immediately after they're created, and so we're not going to talk about them here. Gluons are particles which bind quarks together to form protons, neutrons, and other stuff.

So we're left with the following picture, which should not be taken too seriously. It's only here to show you the levels of structure in an atom. It looks like an atom about as much as a weather map full of jagged lines, giant L's and H's, and a puffy cloud with a golden thunderbolt looks like the sky.

The structures of the atom

Anyway, quarks and gluons aren't made up of anything smaller that we can tell. Which means, that's it. Atoms are made up of electrons and nuclei. Nuclei are made up of protons and neutrons. Protons and neutrons are made up of quarks and gluons.

Where it stops, nobody knows.

Next page
The atom

* If you've seen the classic WKRP in Cincinnati episode in which Venus Flytrap explains the atom, you already have the gist of much of this page, and can continue to the next one if you like. Or, go back to the top of this one.