Inter-particle collisions are frequent and elastic. Total K. E. unchanged by collisions. Only one force acts on particles. Strong repulsive force when they touch. Use of model to describe macroscopic properties.
Pressure is due to the force required to keep the molecule in a fixed volume.
Gas molecules have no volume and no forces acting at long distances. They can easily be made to occupy a smaller volume. To achieve constant P, the volume increases. Dalton s Law . Molecules do not attract or repel therefore the pressure exerted by one type of gas does not affect the pressure exerted by another.