What are atoms made of?

What are atoms made of? — Fifth Grade Class, Knifley, KY

My answer to that question depends on the level of detail you’re interested in. As an example of what I mean by that statement, imagine describing what a simple house is made of. At the coarsest level, you might say that it consists of a floor, a ceiling, four walls, and a roof. At a greater level of detail, you might say that it consists of many boards, some tarpaper, and lots of nails. At a still finer level of detail, you might say that it consists of atoms and molecules, and… you get the point. So it is with atoms. I’ll answer the question at a fairly coarse level of detail, one that’s familiar to many people, and then say a word or two about the next level of detail.

The principal constituents of an atom are protons, neutrons, and electrons. These are three most important subatomic particles; the main building blocks of matter in the same way that wood, bricks, and steel are the major building blocks of houses. Each of these particles has a mass—the measure of their inertia—and two of them, electrons and protons, are electrically charged. Each electron has one unit of negative charge while each proton has one unit of positive charge. Because an atom is normally electrically neutral—its positive and negative charges must balance—it has an equal number of electrons and protons. The number of neutrons in an atom is somewhat flexible.

These particles, electrons, protons, and neutrons, are held together by several types of forces. The protons and neutrons, which are relatively massive, stick to one another at the center of the atom and form a dense object called the atomic nucleus. The particles in the nucleus are held together by the “nuclear” force, which binds together protons and neutrons that are touching one another. This nuclear force is quite strong and is able to overcome the strongly repulsive electromagnetic forces that the protons in the nucleus exert on one another—like electric charges repel one another and the protons are all positively charged. The electrons circulate around the atom’s nucleus, held in place by the strongly attractive electromagnetic forces that protons exert on electrons—opposite electric charges attract one another and the electrons are negatively charged while the protons are positively charged.

The electrons do most of the circulating around the nucleus, rather than the other way around, because they are much less massive than the nucleus. As with the planets around the sun, the less massive objects tend to orbit the more massive objects. At a basic level, you can view an atom as a tiny solar system with its neutrons and protons at the center and its electrons orbiting around this central nucleus. Quantum physics dramatically complicates this picture, but it’s a helpful picture nonetheless.

At the next level of detail, the protons and neutrons themselves have structure—they are built out of yet smaller particles known as quarks. The particles also stick to one another by tossing particles back and forth—particles including photons and gluons. But that is a whole new story.

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