helium might have leaked out by then? The atoms are so small it leaks out through everything.
The below explanation is a good one for rubber, but its the same for metal and glass as well:
The skin of a balloon is like a mesh at the bottom of a ball pit. Imagine the mesh is large enough that every now and then a ball manages to work it’s way through the webbing and escape. Slowly but surely you’re going to run out of balls in the ball pit. But now imagine someone replenishes the pit with a series of new balls. Some were the same size as the old ones, some were ping pong balls, some were marbles, some were BBs. The BBs are going to drop out of the pit pretty quick, and the ping pong balls not all that long after, right? Those bigger balls are going to be in the pit a long time. Air is a mix of molecules, mostly N2, with some O2, and a little bit of CO2, and a tiny smattering of other stuff. Helium is tiny compared to those, and hydrogen smaller still… but hydrogen at least bonds to itself in the form of H2, while Helium is all “noble” and doesn’t bond with anything. The skin of a latex balloon isn’t very good at hanging onto helium or hydrogen. Try a Mylar balloon, you’ll find it lasts a lot longer. This is because the chemical bonds making up mylar are tighter, and also because the mylar ballons aren’t made to stretch, so you aren’t opening up the mesh when you fill the balloon. And even at that, the helium filled mylar balloon will still empty faster than the air filled one, because the same phenomenon is at work. If you want the balloon to stay inflated longer, try filling it with pure N2, because O2 makes up 20% of air and is smaller than N2
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