atoms
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.
an atom
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.
