You are correct. When I say "lighter or heavier" particles, I am referring to the density of the particles, and the appearance of weight given to those particles when they are under pressure.
When you think about a particle like Oxygen (which usually forms a bond with 1 other oxygen atom, forming O2. These two atoms together have 16 neutrons and 16 protons, compared to Hydrogen's 1 proton and 0 neutrons. This makes a standard oxygen molecule more massive, which means it can exert more force on the hydrogen particle than hydrogen can exert upon the O 2 molecule.
That's a good way of describing it.
So Let's try a few definitions.
1. mass = the amount of stuff. We don't need to say anything about what the stuff is, just that you can have more or less of it.
2. distance = length in normal 3 dimensional space.
3. area = distance squared
4. volume = distance cubed
5. time = normal definition of time.
6. density = the amount of mass per unit volume.
7. Pressure = force per unit area.
Now how we define force is the next step, I called it resistance to movement, but I'm not entirely happy with that,
I don't think this can be F=ma, or F=mv....
Any ideas?