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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 helium(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 believed that noble gases couldnt 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. | 
