Answers

2016-01-17T12:38:11+05:30
Helium is the second lightest element in the universe. It has negligible mass than that of air. also, it is the first inert gas which do not take part in any chemical reaction. hence it is used in baloons
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2016-01-17T14:10:27+05:30
For some applications, H2H2 is more appropriate. For example, weather ballons often use hydrogen due to cost, with side benefits being lower density and lower diffusion rates across the membrane; flammability is not a major concern. 
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