The description we have just given is of course incomplete. We must avoid here the crucial questions why only some patterns of motion are possible, why one lowest level is stable and why the electrons never descend into decreasing states of energy, following the attraction of the nucleus. At the same time one should emphasize that a complete explanation of these facts has been given. This explanation makes precise predictions about many of the properties of matter, and we can have complete confidence that, but for the involved mathematical procedure, all ordinary properties of materials could be precisely predicted. The atom has been explained as completely as Newton has explained the motion of planets. To form an idea what an atom is or why two atoms of, let us say, hydrogen are precisely the same, it is not necessary to search for intricate reasons or deep meanings. Two atoms of the same kind are alike as two pawns are for the chess player, except for one little point: in the case of the pawns we do not _care_ about the difference; in the case of the atoms there _is_ no difference. This is a simple statement and it honestly describes a simple situation. The beauty of science is due to the fact that the correct answers to our most interesting questions have turned out to be surprising by their simplicity. In order to understand an atom one must consider the distribution of electrons around one nucleus. In order to understand a molecule one has to consider the distribution of electrons around two or more nuclei. The _chemical_ behavior of an atom is the manner in which it interacts with other atoms, and that means the precise way in which the electrons rearrange themselves when two or more atoms approach each other. The interaction between atoms occurs mainly between their outermost electrons. It may happen that two quite different atoms, containing nuclei of different charges and different numbers of electrons, may nevertheless be similar in the structure of their outermost electrons. In this case the two atoms exhibit similar chemical properties. Examples are lithium with charge 3 and sodium with charge 11; also helium, charge 2 and neon, charge 10. A most important example for our purpose is the set of three chemically similar atoms: calcium, charge 20, strontium, charge 38; and radium, charge 88.