A controversial view is presented and evidence for its validity is supplied.

　　 A hypothesis is presented and possible means of verifying it are suggested.

　　 A recent development is described and then analyzed.

　　 A dispute is summarized and one side defended.

　　One of the simplest and best known kinds of crystal is the ionic salt, of which a typical example is sodium chloride or ordinary table salt 精制食盐). The fundamental components of an ionic salt are ions: atoms or molecules that have become electrically charged by gaining or losing one or more electrons. In forming sodium chloride, for example, sodium atoms give up an electron and chlorine atoms gain an electron . The ions are attracted to one another by their opposite charges, and they stack together compactly, like tightly packed spheres.

　　Recently, scientists at Michigan State University created a new kind of crystal called an electride. In electrides, the anions are completely replaced by electrons, which are trapped in naturally formed cavities within a framework of regularly stacked cations . Electrides are the first examples of ionic salts in which all these anionic sites are occupied solely by electrons.

　　Unlike other types of anions, anionic electrons do not behave as if they were simple charged spheres. In particular, because of their low mass and their tendency to interact with one another over great distances, they cannot be pinned down to any one location. Instead, they wander close to and among the atoms lining the cavity and interact with electrons in nearby cavities, perhaps changing places with them.

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