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元素周期表是反映元素按原子序数（原子核中质子数）的增加顺序排列时，化学和物理性质周期性重现的图表。

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)

这是一项不朽的科学成就，它的发展说明了科学进步所需要的观察、预测和测试之间的基本相互作用。

It is a monumental scientific achievement, and its development illustrates the essential interplay between observation, prediction, and testing required for scientific progress.

在19世纪，科学家们一直在寻找新的元素。

In the 1800’s scientists were searching for new elements.

到19世纪60年代末，已经发现了60多种化学元素，人们对它们的描述化学也有了更多的了解。

By the late 1860’s more than 60 chemical elements had been identified, and much was known about their descriptive chemistry.

人们提出了各种建议，根据化学和物理性质的相似性把这些元素分组。

Various proposals were put forth to arrange the elements into groups based on similarities in chemical and physical properties.

下一步是识别族属性（物理或化学相似性）和原子质量（单个元素原子的测量质量）之间的联系。

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)

在当时已知的元素按照原子质量的递增顺序排列时，就会发现，连续的元素属于不同的化学族，而这个序列中族的顺序是固定的，并且每隔一段时间重复一次。

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.

因此，当写下一系列的元素来开始每个碱金属的新的水平行，相同组的元素则自动填充在元素周期表的垂直列中。

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.

这张表是现代元素周期表的前身。

This table was the forerunner of the modern table.

当德国化学家洛塔尔·迈耶（Lothar Meyer）和俄罗斯化学家德米特里·门捷列夫（Dmitry Mendeleyev）在1869年至1870年首次提出元素周期表时，三分之一的自然产生的化学元素还没有被发现。

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.

然而，两位化学家都有足够的远见，根据他们对周期性物理和化学性质的分析，在应该找到新元素的地方留下了空白。

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.

门捷列夫比迈耶更大胆，他甚至认为，如果测量到的原子质量把一个元素放在表中错误的位置，原子质量就是错误的。

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.

例如，铟的原子质量以前被认为介于砷和硒之间。

Indium, for example, had previously been assigned an atomic mass between those of arsenic and selenium.

因为元素周期表中没有这两种元素之间的空间，所以门捷列夫建议将铟的原子质量改变为一个完全不同的值，填补镉和锡之间的空白。

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.

事实上，后来的工作已经表明，在元素周期表中，元素不应该严格按照原子质量来排序。

In fact, subsequent work has shown that in a periodic table, elements should not be ordered strictly by atomic mass.

例如，在元素周期表中，碲排在碘之前，尽管它的原子质量略大一些。

For example, tellurium comes before iodine in the periodic table, even though its atomic mass is slightly greater.

这种异常是由于“同位素”的相对丰度或每种元素的种类。

Such anomalies are due to the relative abundance of the “isotopes” or varieties of each element.

给定元素的所有同位素质子数相同，但中子数不同，因此原子质量也不同。

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.

给定元素的同位素具有相同的化学性质，但物理性质略有不同。

The isotopes of a given element have the same chemical properties but slightly different physical properties.

我们现在知道是原子序数（原子核内质子的数量），而不是原子质量数（质子和中子数目的总和），决定了化学性质。

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.

门捷列耶夫在另一方面比迈耶走得更远：他预言了六种尚未发现的元素的性质。

Mendeleyev went further than Meyer in another respect: he predicted the properties of six elements yet to be discovered.

例如，铝下方的空隙表明，会发现一种新元素，其性质与铝类似。

For example, a gap just below aluminum suggested a new element would be found with properties analogous to those of aluminum.

门捷列耶夫将这种元素命名为“eka-铝”（eka在梵文中是“下一个”的意思），并预测了它的性质。

Mendeleyev designated this element “eka-aluminum” (eka is the Sanskrit word for “next”) and predicted its properties.

仅仅5年后，一种具有合适原子质量的元素被分离出来，并被发现者命名为镓。

Just five years later an element with the proper atomic mass was isolated and named gallium by its discoverer.

镓的观测性质与门捷列夫对eka-铝的预测之间的密切对应，为周期律提供了有力的支持。

The close correspondence between the observed properties of gallium and Mendeleyev’s predictions for eka-aluminum lent strong support to the periodic law[1].

当元素按照它们的原子序数排列时，元素的大多数属性会定期重新出现。

When elements are arranged in order of their atomic number, most of the properties of the elements reoccur at regular intervals.

1885年，一种也被门捷列夫提前描述的元素eka-硅（eka-silicon）被发现并被命名为锗，这也为门捷列夫提供了额外的支持。

Additional support came in 1885 when eka-silicon, which had also been described in advance by Mendeleyev, was discovered and named germanium.

元素周期表的结构似乎限制了可能元素的数量。

The structure of the periodic table appeared to limit the number of possible elements.

因此，当瑞利勋爵约翰·威廉·斯特拉特（John William Strutt）于1894年发现了一种不符合先前分类方案的气态元素时，人们感到相当惊讶。

It was therefore quite surprising when John William Strutt, Lord Rayleigh, discovered a gaseous element in 1894 that did not fit into the previous classification scheme.

早在一个世纪以前，亨利·卡文迪许（Henry Cavendish）就注意到，当空气中的氧和氮被除去时，会有残余气体存在，但人们还没有认识到它的重要性。

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.

瑞利和威廉·拉姆齐（William Ramsay）一起分离了这种气体（将它从其他物质中分离出来，进入纯净状态），并将它命名为氩气。

Together with William Ramsay, Rayleigh isolated the gas (separating it from other substances into its pure state) and named it argon.

拉姆齐随后研究了天然气矿床中存在的一种气体，发现它是氦，这种元素在太阳中的存在早在太阳光光谱中就被发现了，但在地球上以前还不知道。

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.

瑞利和拉姆齐假设存在一组新的元素，在1898年时，该序列的其他元素（氖、氪和氙）被分离出来。

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.

The word “interplay” in the passage is closest in meaning to

According to paragraph 1, what pattern did scientists notice when the known elements were written in order of increasing atomic mass?

In paragraph 2, what is the author’s purpose in presenting the information about the decision by Meyer and Mendeleyev to leave gaps in the periodic table?

What reason does the author provide for the claim that “Mendeleyev was bolder than Meyer”?

According to paragraph 2, why did Mendeleyev suggest changing the atomic mass of indium?

It can be inferred from paragraph 2 that tellurium comes before iodine in the periodic table even though tellurium’s atomic mass is slightly greater because

The word “abundance” in the passage is closest in meaning to

The phrase “analogous to” in the passage is closest in meaning to

Paragraph 3 suggests that Mendeleyev predicted the properties of eka-aluminum on the basis of

It can be inferred from paragraph 3 that the significance of the discovery of gallium was that it supported which of the following?

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

The word “postulated” in the passage is closest in meaning to

Look at the four squares  that indicate where the following sentence could be added to the passage. 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.  Where would the sentence best fit? Click on a square  to add the sentence to the passage.

Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

Development of the Periodic Table 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.  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. 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. 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. When elements are arranged in order of their atomic number, most of the properties of the elements reoccur at regular intervals. Additional support came in 1885 when eka-silicon, which had also been described in advance by Mendeleyev, was discovered and named germanium. The structure of the periodic table appeared to limit the number of possible elements. It was therefore quite surprising when John William Strutt, 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.