职称英语考试

【单选题】Learnabout Noble Gases(惰性气体)　　1　Have you ever ridden on a balloon Many tourist spots offer balloon rides inorder for people to see the beauty of a place from above.
A、balloon contains anoble gas called helium(氦). Formerly, balloons contained hydrogenbut hydrogen is very flammable and dangerous when uncontrolleD、Therefore, peopleshifted to helium, which is safer. Helium is safe because it has the propertiesof the noble gases.　　2　People once believed that noble gases couldn’t chemically react at all. Forthis reason, they were called inert gases(惰性气体). They werealso listed under Group 0 in theold 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 somenoble gas compounds were discovereD、　　3 　Thegases are elements, which share similar properties. These properties includebeing monoatomic, colorless, odorless, being able to conduct electricity, andhaving low chemical reactivity. Noble gases include Helium, Neon,Argon,Krypton, Xenon and Radon. These are all found in Group 18, in the rightmostcolumn of the periodic table. If you look at the periodic table, you willnotice that these elements are the only ones, which do not have a charge.Helium has the lowest molecular(分子的) weight whileRadon is the heaviest.　　4 　Remember that chemical reactions occurbecause atoms have valence electrons, which are electrons in their outer shell.When the outer shell is “unfilled” or the required number of electrons is notyet complete, the atom is more reactive. Noble gases have a full outer shell,meaning that they have complete electrons in their outer shell. This completenumber varies. For instance, the outer shell of Helium has 2 valence electronswhile the outer shell of Xenon has 8 electrons. Nowadays, there remains to be afew noble gases because of the low chemical reactivity of these said gases.　　　5because of their properties, noble gases have many importantapplications. 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 doctorsuspects that a person’s brain has been damaged, he might request for MagneticResonance Imaging(MRI). MRI allows the doctor to “see” the brain, withoutoperating on the patient.　　23.paragraph 2__ ___　　24.paragraph 3______　　25.paragraph 4_______　　26.paragraph 5________　
A、what is theperiodic table B、What are noblegases？　　
C、What causes the lowchemical reactivity of noble gases D、How were noblegases discovered E、How were noblegases understood in the past 　　F. what are theapplications of 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、reactiveB、lightest　　
C、importantD、completeE、flammable　　F. unnecessary
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