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Tpo 16-2
第4题
When the German chemist Lothar Meyer and(independently) the Russian Dmitry Mendeleyev first introduced the periodictable in 1869-70, one-third of the naturally occurring chemical elements hadnot yet been discovered. Yet both chemists were sufficiently farsighted toleave gaps where their analyses of periodic physical and chemical propertiesindicated that new elements should be located. 【Mendeleyev wasbolder than Meyer】 and even assumed that if a measured atomic mass put an element inthe wrong place in the table, the atomic mass was wrong. In some cases this wastrue. Indium, for example, had previously been assigned an atomic mass betweenthose of arsenic and selenium. Because there is no space in the periodic tablebetween these two elements, Mendeleyev suggested that the atomic mass of indiumbe changed to a completely different value, where it would fill an empty spacebetween cadmium and tin. In fact, subsequent work has shown that in a periodictable, elements should not be ordered strictly by atomic mass. For example,tellurium comes before iodine in the periodic table, even though its atomicmass is slightly greater. Such anomalies are due to the relative abundance ofthe "isotopes" or varieties of each element. All the isotopes of agiven element have the same number of protons, but differ in their number ofneutrons, and hence in their atomic mass. The isotopes of a given element havethe same chemical properties but slightly different physical properties. We nowknow that atomic number (the number of protons in the nucleus), not atomic massnumber (the number of protons and neutrons), determines chemical behavior.
我不能理解为什么选择B而不是D，唯一觉得有可能的，可能就是assumed和realize的区别了。
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发表于 2010-10-10 17:40:54
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