Key concepts
Gas molecules collide constantly and exchange energy. At equilibrium they follow a Maxwell–Boltzmann distribution: many molecules have intermediate energy, fewer are very slow, and a small tail is very energetic.
Mean translational kinetic energy depends only on absolute temperature: three halves multiplied by the Boltzmann constant and temperature. Molecular mass changes speed at the same temperature, but not mean kinetic energy.
Applications and misconceptions
Distributions explain evaporation, reaction rates, diffusion, atmospheric escape, and gas sensors.
Misconception: molecules at one temperature do not all move at one speed. Temperature describes a group average, not an identical speed for every molecule.