air molecules in space Just wondering about escaped stuff in space like air and water molecules. Where would those molecules ultimately end up going?
Considering that there is other gravitational forces in space like planets , suns and black holes.
Is it possible that loose air molecules in space would all collect at the strongest gravitational source?
 A: The more interesting question is what will happen to Earth. Loss of water by UV ionization of water and subduction of $H_2O$ by tectonic action will result in loss of water. On the other hand water and other elements in the aesthenosphere are replenished by volcanoes. There are general losses by atmospheres of planets. However, largely the biggest sequestration of molecules is by tectonic activity, and these are returned by some rate with volcanism.
Once they enter outer space these molecules may end up as interstellar gas. In time a supernova shock wave might gather some of them into the accretion of new stars and planets. 
A: Different gases have different molecular masses and different planets have different thermal environments/ other forces that impart kinetic energy to the atmospheric gases.
So what gases get thrown off into spaces and what gases remain depends on their mass and available energies in the planet's atmosphere; primarily thermal energy. The molecules, like a rocket ship must reach escape velocity to leave the planet's surface. This wiki article Atmospheric Escape provides greater detail on the subject, and an excellent chart that compares escape velocity vs temperature, planet size and what gases are vulnerable to loss.
