Gases are composed of a many particles that behave like hard spherical
objects in a state of constant, random motion.
These particles move in a straight line until they collide with another
particle or the walls of the container.
These particles are much smaller than the distance between particles, therefore
the volume of a gas is mostly empty space and the volume of the gas molecule
themselves is negligible.
There is no force of attraction between gas particles or between the particles
and the walls of the container.
Collisions between gas particles or collisions with the walls of the container
are elastic. That is, none of the energy of the gas particle is lost
in a collision.
The average kinetic energy of a collection of gas particles is dependent
only upon the temperature of the gas.