We know that hydrogen density is less than air. When the hydrogen density is less.. It tries to float in air , by the concept of buoyancy.
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A helium-filled balloon rises in the atmosphere because the weight of the helium gas is less than the weight of the air it has displaced from the volume of the balloon. This property is commonly referred to as Buoyancy. The He acts as the second lightest Lifting gas, Hydrogen being the lightest.