托福化学类阅读文章翻译，Development of the Periodic Table元素周期表的发展
文章词汇words
元素element
族group
原子序数atomic number
同位素isotope
化学性质chemical property
物理性质physical property
原子atom
原子核nucleus
质子proton
中子neutron
电子electron
稀有气体rare gases
碱金属alkali metal
文章翻译translation
一、元素周期表出现的背景
The periodic table is a chart that reflects the periodic recurrence of chemical and physical properties of the elements when the elements are arranged in order of increasing atomic number (the number of protons in the nucleus). It is a monumental scientific achievement, and its development illustrates the essential interplay between observation, prediction, and testing required for scientific progress. In the 1800's scientists were searching for new elements. By the late 1860's more than 60 chemical elements had been identified, and much was known about their descriptive chemistry. Various proposals were put forth to arrange the elements into groups based on similarities in chemical and physical properties. The next step was to recognize a connection between group properties (physical or chemical similarities) and atomic mass (the measured mass of an individual atom of an element). When the elements known at the time were ordered by increasing atomic mass, it was found that successive elements belonged to different chemical groups and that the order of the groups in this sequence was fixed and repeated itself at regular intervals. It was a natural Idea to break up the series of elements at the points where the sequence of chemical groups to which the elements belonged began to repeat itself. Thus when the series of elements was written so as to begin a new horizontal row with each alkali metal, elements of the same groups were automatically assembled in vertical columns in a periodic table of the elements. This table was the forerunner of the modern table. 元素周期表是一个按照原子序数（原子核中的质子数）排列的反映元素化学和物理性质周期性再现的一个图表。它是一个里程碑式的科学成就，它的发展阐明了科学发展中的观察、预测、测试之间的必要关系。19世纪的科学家都在找寻新的元素。到了19世纪60年代晚期，有60多种元素被发现，人们对于他们的化学性质也多有了解。基于这些元素的化学性质和物理性质的相似性，人们提出各种方案来对这些元素进行分类。然后就是找到群族的元素性质（物理或化学性质的相似性）和原子质量（测量的单个元素的原子质量）之间的联系。当人们按照不断增加的原子质量的顺序排列这些已知的元素时，发现连续的元素属于不同的化学群族，群族的顺序在这一序列中是固定的，并按照一定的间隔重复。很自然人们想到在这些元素所在群族的序列开始自我重复的哪一点上分开这一系列的元素。因此，当这一系列的元素以碱金属开始一段水平的一行，同族的元素被自动聚集到元素周期表的垂直的一列当中。这个图表是现在元素周期表的前身。
二、元素周期表和原子质量的关系
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。但是在周期表中，这两个元素之间是没有缝隙的，门捷列夫建议铟的原子质量可以被改变成一个完全不同的质量，填充镉和锡之间的空位。而事实上，后来的研究表明在元素周期表中，元素的顺序并不严格按照原子质量排序。例如在周期表中碲在碘的前面，但是它的原子质量却要轻的多。这种反常现象是由于每种元素的同位素或者变量的广泛存在。所有元素的同位素都具备相同的质子数，但是中子数确不同。同位素具有相同的化学性质，但是物理性质有细微的差异，我们现在知道原子序数（核原子核中的质子数）决定了元素的化学性质，不是原子质量。
三、门捷列夫对未发现元素性质的预测
Mendeleyev went further than Meyer in another respect: he predicted the properties of six elements yet to be discovered. For example, a gap just below aluminum suggested a new element would be found with properties analogous to those of aluminum. Mendeleyev designated this element "eka-aluminum" (eka is the Sanskrit word for "next") and predicted its properties. Just five years later an element with the proper atomic mass was isolated and named gallium by its discoverer. The close correspondence between the observed properties of gallium and Mendeleyev’s predictions for eka-aluminum lent strong support to the periodic law. Additional support came in 1885 when eka-silicon, which had also been described in advance by Mendeleyev, was discovered and named germanium. 门捷列夫在另一个方面上也比Meyer研究的更加深入：他预测了六种还没有被发现的元素的性质。例如铝后面的一个空位暗示这个新元素将与铝的性质类似。门捷列夫将这个元素命名为“类铝”（eka这个词在梵语中的意思是“下一个”）而且还预测了它的性质。仅仅在五年后这一元素的原子质量就被测量出来并独立成为一种元素，它被他的发现者命名为“镓”。镓所表现出的特性和门捷列夫所预测的“次铝”的性质的高度一致性为元素周期律提供了一个强有力的支持。另一个支持的例证是在1885年也是同样由门捷列夫所预测的“次硅”。后来被人们发现命名为锗。
四、元素周期表中稀有气体元素的发现
The structure of the periodic table appeared to limit the number of possible elements. It was therefore quite surprising when John William Strut (Lord Rayleigh), discovered a gaseous element in 1894 that did not fit into the previous classification scheme. A century earlier, Henry Cavendish had noted the existence of a residual gas when oxygen and nitrogen are removed from air, but its importance had not been realized. Together with William Ramsay, Rayleigh isolated the gas (separating it from other substances into its pure state) and named it argon. Ramsay then studied a gas that was present in natural gas deposits and discovered that it was helium, an element whose presence in the Sun had been noted earlier in the spectrum of sunlight but that had not previously been known on Earth. Rayleigh and Ramsay postulated the existence of a new group of elements, and in 1898 other members of the series (neon, krypton, and xenon) were isolated. 元素周期表的框架限定了可能存在的元素的数量。因此当约翰威廉姆斯杜尔特在1894年发现了一种气体元素并不适合于先前的元素分类表格，这是相当令人惊讶的。一个世纪以前亨利卡文迪许已经注意到当氧气和氮气从空气中被分离后仍然有剩余气体存在，但是这种气体的重要性并没有被人们充分意识到。雷利和威廉姆拉姆齐共同分离出了一种气体（将之与其他物质隔离存放于一个纯净的环境）并将它命名为氩。拉姆齐后来研究了一种存在于天然气沉积物中的气体氦。这种元素存在于太阳中，并且早些时候在太阳光谱中出现时就被注意到，但是之前并没有在地球上并没有被发现过。雷利和拉姆齐认为有一组新的元素存在，这组元素中的其他成员也在1898年被成功分离出来（氖，氪，氙）。