In an elastic collision, a light particle bouncing off a much heavier one keeps almost all of its energy — think of a ping-pong ball hitting a bowling ball. An electron is thousands of times lighter than any gas atom, so each collision transfers only a tiny fraction ΔE/E ≈ 4m/M of its energy to the heavy particle. Switch gases and watch the difference: the atoms change size with their true mass, and the recoil scales as 1/M — light helium gets visibly nudged by each hit, heavier nitrogen barely shifts, and heavy argon stands almost perfectly still while the electron ricochets at full speed.
The fraction is the elastic-collision limit ΔE/E = 4mM/(m+M)², which for m ≪ M reduces to about 4m/M. Because that number is so small, an electron must undergo thousands of elastic collisions before it shares its energy with the gas — and in a short-lived or low-power discharge it simply never gets the chance. Real collisions also include inelastic channels (excitation, ionization) that drain electron energy differently; this model isolates the elastic mass effect that sets the two-temperature character of cold plasma. On-screen ion recoil is exaggerated for visibility; the relative recoil between the three gases is kept true to 1/M.