Every mystery is sure to attract some people who wish to solve it. And the mystery of radioactivity was no exception. Scientists in many parts of the world began to search for clues. Little by little, they found them. It wasn't until the 1930's that the last pieces of the puzzle began to fall into place--the structure of the atom was finally clear. But before we talk about the structure of atoms, let's talk about the structure of houses. Many houses are built of wood, shingles, and glass. However, even though the houses are built of the same materials, they may not look anything alike. Some are ranch houses, some are split levels, and some are colonial houses. On the other hand, sometimes a builder puts up a large development in which all the houses are exactly the same. If a new friend tells you that he lives in the Shady Acres development, you can picture his house. It is just like every other house in Shady Acres. Are you wondering what this has to do with atoms? Well, all atoms are built of the same principal materials. [Illustration] They are called _protons_, _electrons_, and _neutrons_. And just as a ranch house never looks exactly like a colonial house, an atom of one element never looks exactly like the atom of another element. [Illustration] But like the Shady Acres houses, every atom of the same element looks exactly like every other. A hydrogen atom looks like every other hydrogen atom. A carbon atom looks like every other carbon atom. But a hydrogen atom never looks like a carbon atom. Atoms are like houses in still another way, too. Even though the building materials used in two houses or two atoms are the same, the finished structure of a house or an atom depends on the way the materials are arranged. If an atom could be made large enough for you to see, you might think you were watching a satellite (or a fleet of satellites) going around and around one or more planets. [Illustration] The planet--or center of the atom--is called the _nucleus_. It is made mostly of protons and neutrons. The little satellites circling around the nucleus are always electrons. Ordinary atoms always have the same number of protons as they do of electrons. This number is called the _atomic number_. No two elements have the same atomic number. It is the number of protons and electrons that tells us what kind of atom it is. When there are two protons in the nucleus and two electrons circling around it, we know that we have an atom of _helium_. If it doesn't have two protons and two electrons, it isn't helium. [Illustration] A helium atom also has two neutrons in its nucleus. Usually, no atom can change its number of protons or electrons and remain the same kind of atom--but it _can_ change its number of neutrons. Atoms of the same element, but with different numbers of neutrons in the nucleus, are called _isotopes_. Some elements have only one isotope, some have as many as eight or ten. [Illustration]