Chemistry - Does gravity affect the trajectory of gas particles?

Solution 1:

Yes gravity pulls on gas molecules. That is why the atmosphere doesn't just float off into space.

The gist is that the time between collisions is very short in the lower atmosphere, and the distances very short. The mean free path at atmospheric pressure is only about 70 nanometers. So the assumption is that gas particles travel in a straight line between collisions.

Solution 2:

To add to previous answers, all molecules and atoms are affected by gravity and so the density of the atmosphere is greater at the surface of the earth compared to higher up, which is why climbing on Everest most climbers take extra oxygen (although, remarkably, it has been done without this aid).

At room temperature gas molecules have an average thermal energy of $3RT/2$ ($R$ is the gas constant $8.314 \pu{ J mol^{-1} K^{-1}}$) and thus they have some kinetic energy and can jostle around in a random manner.

As there is nothing but empty space between one collision of a gas molecule and another, by Newton's law they would travel in straight lines at constant velocity if no force is applied. Now there is a force and this is gravity acting downwards but over the very small distance between collisions in a gas in a room, the effect is negligible. Thus we say that they travel in 'straight lines' but remember that this is only an approximation, but a very good one.

(Notes: The collisions are called 'elastic' which means that no energy is retained within either molecule after the collision compared to that before it. There is also a distribution of velocities, the Maxwell-Boltzmann distribution, or speed the Maxwell distribution).