It's not rising to the top of the atmosphere per se, but rather that atomic helium is so light that at atmospheric temperature a significant percentage of indiv

It's not rising to the top of the atmosphere per se, but rather that atomic helium is so light that at atmospheric temperature a significant percentage of individual atoms are moving at Earth's escape velocity. Usually they bounce into something else, but there's always some lucky few that make their way through the upper atmosphere... and keep going. That's why, for example, Saturn's atmosphere is 25% helium and 75% hydrogen, but Saturn's moon Titan is basically all nitrogen-methane. Which is also the real answer about helium. Hydrogen we have in near-infinite supply by breaking chemical bonds (e.g. in water). Helium doesn't react with anything, so so the small amount trapped in oil and natural gas deposits from Earth's radioactivity over billions of years is all we have. After that... we'd better start investing in outer solar system mining operations, skimming the atmospheres of gas giants. I, for one, look forward to a future working the gas mines.