Unit 5
The Periodic Table
As our picture of the atom becomes more detailed, we find ourselves in a dilemma.当我们对原子了解的越来越详细时，我们发现我们其中处在两难之中己。With more than 100 elements to deal with, how can we keep all this information straight?由于超过100种元素要处理，我们怎样能理顺所有的信息？One way is by using the periodic table of the elements.一个方法是使用元素周期表。The periodic table neatly tabulates information about atoms. 周期表整齐地列出了原子信息的表格。It records how many protons and electrons the atoms of a particular element contain. 它记录了一个具体的元素的原子包含多少质子和电子。It permits us to calculate the number of neutrons in the most common isotope for most elements. 它允许我们为大多数元素计算最常见的同位素中的中子的数量。It even stores information about how electrons are arranged in the atoms of each element. 它甚至储存了每个元素的原子周围是如何安排电子的信息。The most extraordinary thing about the periodic table is that it was largely developed before anyone knew there were protons or neutrons or electrons in atoms. 关于周期表的最杰出的事情是在任何人知道在原子周围有质子、中子或者电子之前被提出来。
Not long after Dalton presented his model for the atom (an indivisible particle whose mass determined its identity ),chemists began preparing listings of elements arranged according to their atomic weights .不久之后道尔顿提出他的原子模型（一个不可分割的粒子的质量决定其特性）科学家开始准备按他们的原子的重量安排元素表。While working out such tables of elements ,these scientists observed patterns among the elements .当规划出这些元素的表，这些科学家观察这些元素的模型。For example ,it became clear that elements that occurred at specific intervals shared a similarity in certain properties .例如，越来越清晰的表明某些特定间隔的元素共享某些相似的性质。Among the approximately 60 elements known at that time ,the fourth and eleventh ,the fifth and twelfth ,and so on .在那个时候大约60个已知元素中,第四和第十一,第五和第十二（拥有相似性质）,等等。
In 1869, Dmitri Ivanovich Mendeleev, a Russian chemist, published his periodic table of the elements. 在1869年，德米特里·伊万诺维奇·门捷列夫，一位俄罗斯化学家，出版了他的元素周期表。Mendeleev prepared his table by taking into account both the atomic weights and the periodicity of certain properties of the elements. 门捷列夫综合考虑元素的原子量和性质的相似性制成了他的元素周期表。The elements were arranged primarily in order of increasing atomic weight. 元素主要按照原子量增加的顺序排列。In a few cases, Mendeleev placed a slightly heavier element with similar chemical properties in the same row. 在一些情况里，门捷列夫把稍微重一点且性质相似的（元素）放在一列中。For example, he placed tellurium (atomic weight = 128) ahead of iodine (atomic weight = 127) because tellurium resembled sulfur and selenium in its properties, whereas iodine was similar to chlorine and bromine. 例如，他把碲(原子量= 128)排在碘(原子量= 127)前面，因为碲在它的特性方面类似硫和硒，而碘类似于氯和溴。
Mendeleev left a number of gaps in his table. 门捷列夫在他的元素周期表中留下了许多空格。Instead of looking upon those blank spaces as defects, he boldly predicted the existence of elements as yet undiscovered. 他没有认为这些空格的出现是周期表的缺陷，而是大胆的预言还有未被发现的元素的存在。Furthermore, he even predicted the properties of some of these missing elements. 而且，他甚至预言一些这些未发现的元素的特性。In succeeding years, many of the gaps were filled in by the discovery of new elements. 在以后的许多年中，许多空格被发现的新元素填入。The properties were often quite close to those Mendeleev had predicted. 性质经常十分接近于门捷列夫已经预言的那些性质。The predictive value of this great innovation led to the wide acceptance of Mendeleev's table. 这个伟大革新的预言价值是门捷列夫的元素周期表被广泛接受。
It is now known that properties of an element depend mainly on the number of electrons in the outermost energy level of the atoms of the element. 现在知道一种元素的性质主要取决于元素原子最外层能级的电子数目。Sodium atoms have one electron in their outermost energy level (the third). 钠原子在它们的最外层能级(第3层)里有一个电子。Lithium atoms have a single electron in their outermost level (the second). 锂原子在他们的最外层能级(第2)里有一个单电子。The chemical properties of sodium and lithium are similar. 钠和锂的化学性质是相似的。The atoms of helium and neon have filled outer electron energy levels, and both elements are inert. 氦和氖的原子充满电子能级，并且两种元素都是不活泼的。That is, they do not undergo chemical reactions readily. 也就是说，他们不容易发生化学反应。Apparently, not only are similar chemical properties shared by elements whose atoms have similar electron configurations (arrangements) but also certain configurations appear to be more stable (less reactive) than others. 显然，不仅是具有相似的电子构造(安排)的原子的元素具有相似的化学性质，而且某些构造看起来比其它（构造）是更稳定(更少活性)的。
In Mendeleev's table, the elements were arranged by atomic weights for the most part, and this arrangement revealed the periodicity of chemical properties. 在门捷列夫的周期表中，大部分元素按照原子量安排，而且这种安排显示了化学性质的周期性。Because the number of electrons determines the element's chemical properties, that number should (and now does)
