托福阅读训练：元素周期表和原子质量的关系.docVIP

托福阅读训练: 元素周期表和原子质量的关系 小马过河为大家准备了“托福阅读训练: 元素周期表和原子质量的关系”， 供各位备考托福的考生们参考使用，来提高自己的托福成绩！免费咨询电话：400-0123-267 　　When the German chemist Lothar Meyer and (independently) the Russian Dmitry Mendeleyev first introduced the periodic table in 1869-70, one-third of the naturally occurring chemical elements had not yet been discovered. Yet both chemists were sufficiently farsighted to leave gaps where their analyses of periodic physical and chemical properties indicated that new elements should be located. Mendeleyev was bolder than Meyer and even assumed that if a measured atomic mass put an element in the wrong place in the table, the atomic mass was wrong. In some cases this was true. Indium, for example, had previously been assigned an atomic mass between those of arsenic and selenium. Because there is no space in the periodic table between these two elements, Mendeleyev suggested that the atomic mass of indium be changed to a completely different value, where it would fill an empty space between cadmium and tin. In fact, subsequent work has shown that in a periodic table, elements should not be ordered strictly by atomic mass. For example, tellurium comes before iodine in the periodic table, even though its atomic mass is slightly greater. Such anomalies are due to the relative abundance of the isotopes or varieties of each element. All the isotopes of a given element have the same number of protons, but differ in their number of neutrons, and hence in their atomic mass. The isotopes of a given element have the same chemical properties but slightly different physical properties. We now know that atomic number (the number of protons in the nucleus), not atomic mass number (the number of protons and neutrons), determines chemical behavior. 　　当德国化学家迈耶和(彼此独立的)俄国的门捷列夫在1869年和1870年第一次引入元素周期表这一概念时，自然界还有三分之一的化学元素没被发现。然而根据周期性的物理和化学性质的标示，两位化学家都富有预见性的在他们所分析的新元素应该的位置上留下空位。门捷列夫比迈耶更加大胆，他甚至假定如果根据测定的原子质量把一个元素放置在元素周期表中显示的是错误的位置，那么这个元素的原子质量也是错的。在某些情况下，这个假设是对的。以金属铟为例，先前测量出的铟的原子质量在砷和硒之间。但是在周期表