Balloons from kids’ birthday parties often hang around for weeks. But the helium balloon is often on the floor, largely deflated, by the next morning.
Balloons are made from very long stretchy molecules, whose structure is a tangled mass of strands, which twist around each other like spaghetti on a plate. The long molecular strands cannot pack very tightly together, and have many channels through which the helium can spread out.
When the balloon is inflated, the molecules stretch. The structure becomes more open as the walls become thinner, so molecules moving from inside the balloon travel a shorter distance through wider channels as they spread through the walls.
Although they may stay inflated for several days, they always deflate quicker than the same balloon filled with air. Air is a mixture of oxygen and nitrogen molecules, which are much larger and heavier than helium atoms. Small light atoms can diffuse through tiny pores in stretched rubber much more easily than larger heavier molecules.
However, helium balloons don’t completely deflate when helium atoms diffuse out of the balloon, because some air molecules also move into the balloon. In fact, if we fill a balloon with something that has molecules bigger and heavier than nitrogen and oxygen, then air diffuses into the balloon, and it slowly inflates!