The smallest particles known to man are called *atoms*. They may be said to be the *"building blocks"* of matter. All substances are composed of atoms. An atom is composed of even smaller particles: *protons, electrons, and neutrons.*

The protons and neutrons are located in a tiny nucleus within the atom. The electrons are tiny particles that *revolve* around the nucleus. Most of the atom is *empty* space. If the protons and neutrons in the nucleus of an atom were as small as the head of a pin, the electrons would be roughly one-quarter of a mile away.

We don't know what atoms look like, because no one has ever seen one, but scientists have devised certain *models* and symbols that help us to understand their composition and behavior.

The atomic *models* show the nucleus as a tiny sphere surrounded by one or more electron shells. Each shell consists of one or more orbits. The electrons move around the orbits at speeds averaging 1,000 miles per second.

A solid such as a *piece of wood* is composed of many molecules tightly grouped together; liquid contains molecules which are looser-spaced; and a gas contains molecules which have comparatively great distances between them.