This helium atom is made up of four electrons and four protons. Right at the center there is a small closely crowded group which contains all the protons and two of the electrons. The other two electrons play around quite a little way from this inner group. It will make our explanations easier if we learn to call this inner group "the nucleus" of the atom. It is the center of the atom and the other two electrons play around about it just as the earth and Mars and the other planets play or revolve about the sun as a center. That is why we shall call these two electrons "planetary electrons." There are about ninety different kinds of atoms and they all have names. Some of them are more familiar than hydrogen and helium. For example, there is the iron atom, the copper atom, the sulphur atom and so on. Some of these atoms you ought to know and so, before telling you more of how atoms are formed by protons and electrons, I am going to write down the names of some of the atoms which we have in the earth and rocks of our world, in the water of the oceans, and in the air above. Start first with air. It is a mixture of several kinds of gases. Each gas is a different kind of atom. There is just a slight trace of hydrogen and a very small amount of helium and of some other gases which I won't bother you with learning. Most of the air, however, is nitrogen, about 78 percent in fact and almost all the rest is oxygen. About 20.8 percent is oxygen so that all the gases other than these two make up only about 1.2 percent of the atmosphere in which we live. [Illustration: Pl. II.--Bird's-eye View of Radio Central (Courtesy of Radio Corporation of America).] The earth and rocks also contain a great deal of oxygen; about 47.3 percent of the atoms which form earth and rocks are oxygen atoms. About half of the rest of the atoms are of a kind called silicon. Sand is made up of atoms of silicon and oxygen and you know how much sand there is. About 27.7 percent of the earth and its rocks is silicon. The next most important kind of atom in the earth is aluminum and after that iron and then calcium. Here is the way they run in percentages: Aluminum 7.8 percent; iron 4.5 percent; calcium 3.5 percent; sodium 2.4 percent; potassium 2.4 percent; magnesium 2.2 percent. Besides these which are most important there is about 0.2 percent of hydrogen and the same amount of carbon. Then there is a little phosphorus, a little sulphur, a little fluorine, and small amounts of all of the rest of the different kinds of atoms.