GRE阅读模拟在线练习附答案二百七十二

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　　One of the simplest and best known kinds of crystalis the ionic salt, of which a typical example issodium chloride, or ordinary table salt. Thefundamental components of an ionic salt are ions: atoms or molecules that have become electricallycharged by gaining or losing one or more electrons. In forming sodium chloride, for example, sodiumatoms give up an electron (thereby becoming, positively charged) and chlorine atoms gain an electron (thereby becoming negativelycharged). The ions are attracted to one another by their opposite charges, and they stacktogether compactly, like tightly packed spheres.

　　Recently, scientists at Michigan State University created a new kind of crystal called anelectride. In electrides, the anions (negative ions) are completely replaced by electrons, whichare 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 areoccupied solely by electrons.

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

　　The properties of an electride depend largely on the distance between the cavities that holdtrapped electrons. When the trapped electrons are far apart, they do not interact strongly, andso behave somewhat like an array of isolated negative charges. When they are closertogether, they begin to display properties associated with large ensembles of identicalparticles. 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 lessfree 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 theanionic cavities and their relation to the surrounding cations. The resulting properties maymake it possible for electrides to become a basis for economically useful new materials anddevices. For instance, because the electrons in some electrides are very weakly bound, thesecrystals could be effective as photosensitive detectors, in which an impinging photon liberatesan electron, resulting in a small electric current. The same weak binding could also makeelectrides useful in solar-energy converters and as cathodes in batteries. One obstacle is thetendency of electrides to decompose through reaction with air and water. Researchers areseeking ways to increase their stability.

　　The passage is primarily concerned with discussing

　　A.a way to isolate electrons

　　B.the characteristics of a new kind of crystal

　　C.the structure of an ionic salt

　　D.commercial uses for electrides

　　E.the properties of ions

　　In the first paragraph, the author is primarily concerned with

　　A.introducing a variant on the standard atomic theory

　　B.describing how chlorine atoms can become negatively charged

　　C.describing some early research at Michigan State University

　　D.presenting the identifying properties of an electride

　　E.providing background for the technical discussion to follow

　　It can be inferred from the passage that the differences between the behavior ofanionic electrons and normal anions result from which of the following features ofelectrons, as compared to normal anions?

　　I. The much lower mass of electrons

　　II. The much greater tendency of electrons to interact with one another over large distances

　　III. The much greater likelihood of electrons to remain trapped in naturally formed anioniccavities

　　A.I only

　　B.II only

　　C.I and II only

　　D.I and III only

　　E.II and III only

　　According to the passage, the defining characteristic of an electride is which of thefollowing?

　　A.Its positive ions are of particularly low mass.

　　B.Its ions possess identical electrical charges.

　　C.It contains a framework of regularly stacked ions.

　　D.Its ions demonstrate strong mutual attraction.

　　E.Its negative ions consist solely of electrons.

　　It can be inferred from the passage that anions behaving as “simple charged spheres” (line 22) could be expected to

　　A.readily lose electrons and become positively charged

　　B.move freely in and out of their cavities

　　C.respond to photons by liberating electrons

　　D.stack with other anions to create a regular framework

　　E.remain fixed relative to their cations

　　It can be inferred from the passage that an electride behaves most like a normal ioniccrystal when the electride has which of the following features?

　　A.The anionic cavities are widely separated.

　　B.All of the trapped electrons are able to delocalize.

　　C.The trapped electrons are liberated by impinging photons.

　　D.The ions are tightly packed together.

　　E.Most of the cations have lost their electrical charge.

　　With which of the following statements regarding electrides would the author mostlikely agree?

　　A.They have proven themselves to be of great commercial value.

　　B.Their future commercial value is promising but uncertain.

　　C.They are interesting but of no practical value.

　　D.They have commercial value mainly in solar-energy applications.

　　E.Their principal importance will lie in scientific research.

　　正确答案:B E C E E A B

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