学位英语

第3部分 概括大意与完成句子

Learn about Noble Gases（惰性气体）

1 Have you ever ridden on a balloon? Many tourist spots offer balloon rides in order for people to see the beauty of a place from above. A balloon contains a noble gas called hellum（氦）. Formerly, balloons contained hydrogen but hydrogen is very flammable and dangerous when uncontrolled. Therefore, people shifted to helium, which is safer. Helium is safe because it has the properties of the noble gases.

2 People once belleved that noble gases couldn't chemically react at all. For this reason, they were called inert gases（惰性气体）. They were also listed under Group 0 in the old periodic table because scientists believed that the gases have zero valence（价）electrons in their outer shell. This was later proven to be untrue when some noble gas compounds were discovered.

3 The gases are elements, which share similar properties. These properties include being monoatomic, colorless, odorless, being able to conduct electricity, and having low chemical reactivity. Noble gases include Helium, Neon, Argon, Krypton, Xenon and Radon. These are all found in Group 18, in the rightmost column of the periodic table. If you look at the periodic table, you will notice that these elements are the only ones, which do not have a charge. Helium has the lowest molecular（分子的）weight while Radon is the heaviest.

4 Remember that chemical reactions occur because atoms have valence electrons, which are electrons in their outer shell. When the outer shell is "unfilled" or the required number of electrons is not yet complete, the atom is more reactive. Noble gases have a full outer shell, meaning that they have complete electrons in their outer shell. This complete number varies. For instance, the outer shell of Helium has 2 valence electrons while the outer shell of Xenon has 8 electrons. Nowadays, there remains to be a few noble gases because of the low chemical reactivity of these said gases.

5 because of their properties, noble gases have many important applications. They are widely used in medicine and industries. For instance, liquid Helium is used for superconducting magnets（磁体）. These magnets are very important in physics and medicine. When a doctor suspects that a person's brain has been damaged, he might request for Magnetic Resonance Imaging(MRI). MRI allows the doctor to "see" the brain, without operating on the patient.

23. paragraph 2

24. paragraph 3

25. paragraph 4

26. paragraph 5

A. How were noble gases discovered?

B. what is the periodic table?

C. what are the applications of noble gases?

D. How were noble gases understood in the past?

E.What causes the low chemical reactivity of noble gases?

F. What are noble gases?

27. Noble gases are not very chemically .

28. Among the elements of noble gases Helium is the .

29. The required number of electrons in noble gases'outer shell is .

30. MRI may make operating on the patient .

A. complete B. reactive C.unnecessary D. flammable E. important F. lightest

23  E  根据该段第一句话：People once believed that noble gases couldn’t chemically react at all.

24  A  本段开头介绍了noble gases的相同点，紧接着作者又列出了noble gases包括的气体以及它们的特性。

25  C  该段第一句和最后一句中的chemical reaction和chemical reactivity点出了该选项。

26  F  该段开头...noble gases have many important applications是本段的主题句。

27  B  第二段第一句中的react可以用be reactive替换。

28  A  根据第三段最后一句：Helium has the lowest molecular（分子的）weight．

29  D  根据第四段第三句：...meaning that they have complete electrons in their outer shell．

30  F  根据本文最后一句：MRI allows the doctor to "see" the brain,without operating on the

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