We will assume that in an ideal gas, molecular attractive forces are not just small, but identically zero. The average kinetic energy of the molecules does not change with time. The molecules bounce and bounce but, on average, do not slow down as long as the temperature of the gas remains constant. At a given temperature the molecules of all species of gas, no matter what size shape or weight, have the same average kinetic energy. If the absolute temperature of a gas is doubled , the average kinetic energy of its molecules is doubled.
Collisions change individual molecular speeds but the distribution of speeds remains the same.
At the same temperature, lighter gases move, on average, faster than heavier gases. This Graph shows the Distribution of Speeds for Gases, i.e.the fraction of the sample of gas molecules that have a given speed is shown by the height of the curve above the speed axis. There are no molecules exactly at rest. At higher temperatures at greater fraction of the molecules are moving at higher speeds. This is important for activated chemical processes, reactions.