[jkqwzsoo]

Umm, not quite.

H = 1

C = 12 (6 protons, 6 neutrons)

O = 16 (8 protons, 8 neutrons)

H2O = 18

O2 = 32

CO2 = 44

Also, the connection between atmosphere retention and gravity isn't quite so direct. If you positioned a molecule of H2O (or even H2) directly above the surface of the moon with no net velocity, it will fall to the ground. However, once it hit the ground, it would thermalize with the surface and could be ejected in any direction at a range of speeds (or react with the surface and never leave).

The simple/approximate method to compute the stability of the atmosphere is to compute the escape velocity for the body and the Maxwell-Boltzmann distribution of speeds for the gas (which does not depend on gravity). You then integrate the probability distribution P(v) (probability of velocity v) for speeds greater than the escape velocity to infinity. This might result in a case where all gases eventually make their way away from the body, but heavier gases simply take longer.

[Arnavion]

Not to mention, the gravitational acceleration felt by a body is independent of its mass, so the atomic weight of the gas molecules makes no difference to the acceleration they experience anyway.

The reason "lighter" gases float above "heavier" gases under gravity is not because of different gravitational acceleration but because of buoyancy.