Why do the helium filled balloons rise? If we look at the periodic table, we see that He (helium) is one of the lightest elements, with an atomic weight of 4, much smaller than O (oxygen), which has an atomic weight of 16. This is why helium balloons float.

 

H (hydrogen) is four times as light as helium. Why don’t they use hydrogen? The reason is that hydrogen can explode and burn. Helium is a noble gas, is inert, and so will not burn.

 

Speaking of hydrogen, how much hydrogen is there in the air? Answer: none. If there were any, it would float away.