1、Periodic tableHumans have always known about certain elemental substances, such as gold and broken silver, that occur in nature and cannot be broken down into other substances. Other chemical elements were discovered gradually beginning in 1669, when a German merchant, Hennig Brand, accidentally fou
2、nd phosphorus while attempting to find a way to create gold out of more common metals. By 1809, the number of known elements had increased to 47. Chemists studying the elements began to notice patterns in the way chemicals reacted.The first modern chemistry textbook was written in 1789 by Antoine-La
3、urent de Lavoisier. He listed the known elements, which he classified into metals and nonmetals. But his listing omitted the gases and included some things that were not substances, such as light and calories. Thus, his list was not accepted as an accurate organizing system.An important trait of ele
4、ments is their periodicity: similarly acting elements tend to occur at regular intervals when arranged by their atomic weight(the number of protons plus neutrons in the nucleus). A French geologist, Alexandrr-Emile Beguyer de Chancourtois, was the first to notice this periodicity. He designed a prec
5、ursor to the periodic table, the telluric helix. That system arranged the elements on a spiral-shaped cylinder in order of their atomic weight. The elements with similar physical properties appeared vertically on the cylinder. But because Chancourtoiss 1862 paper used geological terms and did not in
6、clude drawings, his periodic system was not accepted by those in the field of chemistry.As new elements were discovered through the first half of the 1800s, chemists confirmed the regular repetition of the physical properties classify the elements in order to reflect this periodicity. In 1863， an En
7、glish chemist， John Newlands, divided the 56 known elements into groups, each sharing the same characteristics. As each froup seemed to contain eight elements, he referred to his system as the Law of Octaves, after the eight keys in an octave on the piano. But Newlandss idea was ridiculed and his th
8、eory dismissed. Not until 1919 did it become accepted that the elements should be grouped by eights.But the underlying concept of the periodicity of the elements was adapted to better effect in 1869 by the Russian chemist Dmitri Mendeleev. His great breakthrough was to see that he two key characteri
9、stics of an element-atomic weight and atomic number (the number of protons in the nucleus)-could be combined in a single table. His table came to be called the periodic table.Mendeleevs table was inspired by the card game of solitaire, in which cards are arranged horizontally by suit and vertically
10、by number. Borrowing the same organizing patterns, Mendeleev arranged the elements in groups of seven. He grouped them horizontally by their atomic number in ascending order and vertically by their similar qualities in groups of seven. Thus, similar metals such as gold, silver, and copper appear in
11、the same vertical column. Similarly reacting gases such as helium, argon, and neon appear in another column. The most common elements(hydrogen, helium, lithium) have lower atomic numbers and thus appear near the beginning of the table in the first horizontal row. The rarest elements (uranium and plu
12、tonium) have the greatest number of protons in theire nuclei. They are ordered near the end of the table.In Mendeleevs time, only 63 elements had been discovered. The brilliance of his periodic table, however, was that it predicted that new elements would be found to fit into the missing slots in hi
13、s original table. Today the table shows 120 elements, 82 naturally occuriring and 28 created in the laboratory. Scientists believe that many more will be found. The periodic table has been called “the most elegant organizational chart ever devised.”Mendeleevs chart remains valid today. But it has be
14、en modified by the continual discovery and manufacture of new elements. And in 1914,Henry Mosely discovered a relationship between an elements X-ray wavelengthe and its atomic number. Thus, he rearrandged the elements by electrica charge. Another important improvement was suggested by Glenn T. Seaborg in 1945: the addition of vertical group of certain heavy elements called the actinide series.10bj_toefl_jc_yd_5_2 以及 10bj_toefl_jc_yd_5_3 均使用到上述文字
