I was given a Helium balloon by a friend for our anniversary and its gradually losing Helium as the days go by. Helium can escape because it has the second smallest atom in the periodic table, and can diffuse through the skin of the balloon. Helium is also inert so unlike Hydrogen that has the smallest atoms, it won’t burn. (Hydrogen on the other hand is very reactive and burns with Oxygen in the atmosphere to make water).
The balloon has gradually sunk in the air as its buoyancy has reduced. We gradually cut off the ribbon on it and as that reduced its weight it flew higher. Its also following air currents and drifted around the house. Rather like a drop of water floating in the international space station the balloon has drifted up and down the stairs. When it gets above a radiator it ascends, and when it drifts out of the warm air it descends again. At the moment it’s bobbing around the living room, like one of the giant bubbles in the 1960s series ‘the prisoner’….