The main assumption of the kinetic theory of gas is that gas molecules occupy negligible space inside the container. This is completely explained in chapter 12 of class 11 Physics. Molecules follow a linear motion as they collide with the walls of the container and with each other. They exert no force of attraction or repulsion on each other, except during a collision. Collisions between gas molecules are completely elastic, i.e. there is no loss or gain of kinetic energy in the process and the total kinetic energy remains constant. The collision of gas molecules with the walls of the container creates gas pressure.