Pl. II.–Bird’s-eye View of Radio Central
(Courtesy of Radio Corporation of America).
11The 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.
Sea water is mostly oxygen and hydrogen, about 85.8 percent of oxygen and 10.7 percent of hydrogen. That is what you would expect for water is made up of molecules which in turn are formed by two atoms of hydrogen and one atom of oxygen. The oxygen atom is about sixteen times as heavy as the hydrogen atom. However, for every oxygen atom there are two hydrogen atoms so that for every pound of hydrogen in water there are about eight pounds of oxygen. That is why there is about eight times as high a percentage of oxygen in sea water as there is of hydrogen.
Most of sea water, therefore, is just water, that is, pure water. But it contains some other substances as well and the best known of these is salt. Salt is a 12substance the molecules of which contain atoms of sodium and of chlorine. That is why sea water is about 1.1 percent sodium and about 2.1 percent chlorine. There are some other kinds of atoms in sea water, as you would expect, for it gets all the substances which the waters of the earth dissolve and carry down to it but they are unimportant in amounts.
Now we know something about the names of the important kinds of atoms and can take up again the question of how they are formed by protons and electrons. No matter what kind of atom we are dealing with we always have a nucleus or center and some electrons playing around that nucleus like tiny planets. The only differences between one kind of atom and any other kind are differences in the nucleus and differences in the number and arrangement of the planetary electrons which are playing about the nucleus.
No matter what kind of atom we are considering there is always in it just as many electrons as protons. For example, the iron atom is formed by a nucleus and twenty-six electrons playing around it. The copper atom has twenty-nine electrons as tiny planets to its nucleus. What does that mean about its nucleus? That there are twenty-nine more protons in the nucleus than there are electrons. Silver has even more planetary electrons, for it has 47. Radium has 88 and the heaviest atom of all, that of uranium, has 92.
We might use numbers for the different kinds of 13atoms instead of names if we wanted to do so. We could describe any kind of atom by telling how many planetary electrons there were in it. For example, hydrogen would be number 1, helium number 2, lithium of which you perhaps never heard, would be number 3, and so on. Oxygen is 8, sodium is 11, chlorine is 17, iron 26, and copper 29. For each kind of atom there is a number. Let’s call that number its atomic number.
Now let’s see what the atomic number tells us. Take copper, for example, which is number 29. In each atom of copper there are 29 electrons playing around the nucleus. The nucleus itself is a little inner group of electrons and protons, but there are more protons than electrons in it; twenty-nine more in fact. In an atom there is always an extra proton in the nucleus for each planetary electron. That makes the total number of protons and electrons the same.
About the nucleus of a copper atom there are playing 29 electrons just as if the nucleus was a teacher responsible for 29 children who were out in the play yard. There is one very funny thing about it all, however, and that is that we must think of the scholars as if they were all just alike so that the teacher couldn’t tell one from the other. Electrons are all alike, you remember. All the teacher or nucleus cares for is that there shall be just the right number playing around her. You could bring a boy in from some other play ground and the teacher couldn’t tell that he was a stranger but she would 14know that something was the matter for there would be one too many in her group. She is responsible for just 29 scholars, and the nucleus of the copper atom is responsible for just 29 electrons. It doesn’t make any difference where these electrons come from provided there are always just 29 playing around the nucleus. If there are more or less than 29 something peculiar will happen.
We shall see later what might happen, but first let’s think of an enormous lot of atoms such as there would be in a copper wire. A small copper wire will have in it billions of copper atoms, each with its planetary electrons playing their invisible game about their own nucleus. There is quite a little distance in any atom between the nucleus and any of the electrons for which it is responsible. There is usually a greater distance still between one atomic group and any other.