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  3. One of the simplest and best known kinds of crystal is the ionic salt (离子盐), of which a typical example is sodium chloride, or o

One of the simplest and best known kinds of crystal is the ionic salt (离子盐), of which a typical example is sodium chloride, or o

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问题     One of the simplest and best known kinds of crystal is the ionic salt (离子盐), of which a typical example is sodium chloride, or ordinary table salt. The fundamental components of an ionic salt are ions: atoms or molecule 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 (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 panicles. When they are still closer, the ensemble properties dominate and the electrons "delocalize": they are no longer lightly bound within individual cavities but are more or less free to pass through the spaces within the frame-work 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.
It can be inferred from the passage that an electride behaves most like a normal ionic crystal when electride has which of the following features?
选项 A、The ions are tightly packed together.
B、The anionic cavities are widely separated.
C、All of the trapped electrons are able to delocalize.
D、The trapped electrons are liberated by impinging photons.
答案B
解析 本题适合采用排除法。A项所有的离子都簇拥一团，第一段的最后一句话有提到此信息，但是这只是针对正常离子而言，与阴离子电子无关。C项说所有的被俘电子都能脱位，文中第四段的最后一句话提到此信息，但这只是在特定形态下电子才具有的性质。D项所有的被俘电子由光子冲击解脱，文中提及“被俘电子由光子冲击”时只是为了介绍晶体的一种运用方式，与题意无关。由文中第一段可知普通的离子晶体结构稳定，由最后两段可知电子晶体结构不稳定，它的特性取决于空穴之间的距离：距离远时电子之间的相互作用不强，因而在某种程度上表现为一组相互孤立的负电荷，此时它的结构较稳定，故答案为B项。
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