Atom

Everything we can see around us, including our own bodies, is made of atoms. They're the things that combine together to make molecules, which in turn make up everything from tables to turkeys. The ancient Greeks invented the term "atom" to mean something that is as small as possible, and can't be broken down even further. But, as modern physicists have shown, there is something even smaller than the atom. In fact, there are lots of things. These are called subatomic particles. (In fact, subatomic means "smaller than the atom.") Each atom has an inner structure made of many smaller particles, and some of those particles have an inner structure of even smaller particles. The differences among the inner structures of atoms cause the differences between elements like hydrogen, gold, neon and lead. Let's take a look inside a typical atom.

The picture at top left may look familiar -- it's the way atoms are often depicted. The blue lines represent particles called electrons, which orbit the yellow center, called the nucleus (the plural of nucleus is nuclei). The electrons aren't important, so let's give attention to the yellow nucleus. In the center of the picture, you can see a magnified nucleus. And, you can see that a nucleus has many things inside it! In general, the particles inside the nucleus are called nucleons. But each kind of nucleon has its own name. The red circles represent protons, and the blue circles are neutrons. There are even smaller particles inside the protons and neutrons; the green circles are quarks, while the yellow squiggles represent particles called gluons. Just like Elmer's glue holds paper together, gluons hold quarks together. You can also see arrows inside the quarks -- these show the type of quark. Protons always have two "up" quarks and one "down" quark, while neutrons have two "down" quarks and an "up" quark. This atom has nine protons, nine neutrons, and nine electrons. But atoms can have many different combinations of particles. A hydrogen atom, for example, has just one proton and one electron. A typical gold atom has 79 protons, 79 electrons, and 118 neutrons. That's a heavy atom! Now, physicists use only the nuclei of atoms -- they remove the electrons. Whenever an atom has fewer electrons than protons, it's called an ion.

How small are atoms and subatomic particles? If you tried to measure them in inches or centimeters with a ruler, you'd have a lot of zeros to deal with! For example, a typical atom is 0.000000001 meters across -- that's one billionth of a meter!