Helium is the gas that fills up balloons, not nitrogen gas. Helium is a non-flammable, non-toxic, inert gas that is lighter than air. This means that it will rise when released into the atmosphere, which is why it is used to fill balloons. Nitrogen gas, on the other hand, is a diatomic gas that makes up about 78% of the Earth's atmosphere. It is not lighter than air, so it will not cause balloons to rise.