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  3. One of the simplest and best known kinds of crystal is the ionic salt, of which a typical example is sodium chloride, or ordinar

One of the simplest and best known kinds of crystal is the ionic salt, of which a typical example is sodium chloride, or ordinar

admin2014-12-26  50
问题     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. The fundamental components of an ionic salt are ions: atoms or molecules that have become electrically charged by gaining or losing one more electrons. In forming sodium chloride, for example, sodium atoms give up an electron(thereby becoming positively charged)and chlorine atoms gain an electron(thereby becoming negatively charged). 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(negative ions)are completely replaced by electrons, which are trapped in naturally formed cavities within a framework of regularly stacked cations(positive ions). 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.
    The properties of an electride depend largely on the distance between the cavities that hold trapped electrons. When the trapped electrons are far apart, they do not interact strongly, and so behave somewhat like an array of isolated negative charges. When they are closer together, they begin to display properties associated with large ensembles of identical particles. When they are still closer, the ensemble properties dominate and the electrons "delocalize": they are no longer tightly bound within individual cavities but are more or less free to pass through the spaces within the framework of positive ions.
    By synthesizing electrides from a variety of materials, one can vary the geometry of the anionic cavities and their relation to the surrounding cations. The resulting properties may make it possible for electrides to become a basis for economically useful new materials and devices. For instance, because the electrons in some electrides are very weakly bound, these crystals could be effective as photosensitive detectors, in which an impinging photon liberates an electron, resulting in a small electric current. The same weak binding could also make electrides useful in solar energy converters and as cathodes in batteries. One obstacle is the tendency of electrides to decompose through reaction with air and water. Researchers are seeking ways to increase their stability.
According to the text, the defining characteristic of an electride is which of the following?
选项 A、Its positive is of particularly low mass.
B、Its ions possess identical electrical charges.
C、It contains a framework of regularly stacked ions.
D、Its negative ions consist solely of electrons.
答案D
解析 细节题。根据“electride”定位到第二段In electrides，the anions(negativeions)are completely replaced by electrons，which are trapped in naturally formedcavities within a framework of regularly stacked cations(positive ions)． Electrides arethe first examples of ionic salts in which all these anionic sites are occupied solely byelectrons．“在电子晶体中，阴离子(即带负电荷的离子)全部被电子替代，这些电子被困在自然形成的空穴中，而空穴位于规则堆砌的阳离子(即带正电荷的离子)的框架内。电子晶体是一种阴离子空间全被电子占据的离子盐。“可知电子晶体的阴离子完全是由电子构成的，D选项和原文表述一致，所以答案选D。
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