Question

As a gas-filled balloon ascends in atmosphere, its weight

Answer 1

Their weights are unique, so they are not "attracted"/pulled in by gravity similarly.

The point just is that it is less demanding for helium atoms to climb than for air atoms. In the event that you shake the container with full effort, the stones may hop a bit while the plastic balls can hop considerably higher.

So all things considered, the helium atoms will move significantly higher upwards, and when they do that, some oxygen particles will take their position which was taken by them previously. As a result, they have another area higher up, and the same happens.