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Learn About Noble Gases（惰性气体）
the beauty of a place from above.A balloon contains a noble gas called helium（氦）.Formerly,balloons con-
tamned 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.called inert gases(隋性气体）.They were also listed under Group 0 in the old periodic table because scien-
tists 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.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 right most
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.in their outer shell.When the outer shell is"unfilled"or the required number of electrons is not yet com-
plete,the atom is more reactive.Noble gases have a full outer shell,meaning that they have complete elec-
trons 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.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. Paragraph 5__________
tamned 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.
tists 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.
include helium,neon,argon,krypton,xenon and radon.These are all found in Group 18,in the right most
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.
plete,the atom is more reactive.Noble gases have a full outer shell,meaning that they have complete elec-
trons 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.
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. Paragraph 5__________
参考答案: F
参考解析: 文章的第二段主要讲述了过去人们对惰性气体的看法。人们曾认为这些惰性气体不 会产生任何化学反应，故选E。
文章第三段主要讲的是惰性气体的性质、特点以及种类，故选B。
文章第四段从一般的化学反应发生的原理着手，解释惰性气体化学反应性低的原因： 惰性气体外层电子数目完整，不易发生反应，故选C。
由本段“Because of their properties, noble gases have many important applications.”一句可 知，本段主要是介绍惰性气体的应用实例，故选F。
文章第三段主要介绍了惰性气体的特点，由本段第二句中的“and having iow chemical reactivity”可知，应选A。
由文章第三段最后一句“Helium has the lowest molecular weight while radon is the heavi- est.”可知，应选B。
由文章第四段第三句“Noble gases have a full outer shell , meaning that they have complete electrons in their outer shell.”可知，惰性气体外层电子数目完整，故应选D。
由文章最后一段的最后一句话“MRI allows the doctor to ' see ' the brain , without operat- ing on the patient.”可知，MRI（磁共振成像）可以使医生在不进行开刀手术的前提下看到大脑 内部情况，故选F。
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