Why is helium lighter than CO2?

Well, isn't that just a happy little question! Helium is lighter than CO2 because it has a lower Atomic Mass. You see, helium is made up of smaller atoms than carbon dioxide, so it floats up above it like a fluffy little cloud in the sky. Just like how we can't help but smile when we see a balloon floating gently in the air, helium can't help but rise above CO2.