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Matter is composed of atoms consisting of positively charged nuclei and negative electrons. These electrons occur in shells, and the periodic nature of chemical properties of atoms as the atomic weight increases is a reflection of the fact that the chemical behavior of an atom depends largely upon the number of electrons in the outermost shell. In some parts of the periodic table (see Table 5 of Appendix A), electrons occupy places in an outer shell before an inner shell is completely filled; it is then observed that a number of different elements have very similar chemical properties. The same number of electrons lies in the same outermost shell of these different atoms, but the inner shells contain different numbers of electrons. The chemical similarity of the rare-earth elements, atomic numbers 57 to 71, may be explained on this basis. A similar state of affairs exists in the group of elements of atomic number 26 (iron), 27 (cobalt), and 28 (nickel), all of which contain 2 electrons in their outermost shell but have 6, 7, and 8 electrons respectively, in their next inner shell, where 10 electrons are required to fill that shell. As a consequence the electrons in the unfilled shell of one atom may exert an important influence on the electrons of the unfilled shell of a properly spaced adjacent atom in a crystal. The ferromagnetism of iron, nickel, and cobalt is explicable in terms of the electronic configuration of these partially filled shells.