At ordinary temperatures and pressures, both hydrogen and helium are gases (upper left). Individual particles move freely, are far apart on the average, and are independent of one another except when they collide. Their energy of motion is sufficiently greater than the van der Waals attractions that when they do collide, they rebound rather than stick together. Hydrogen and helium gases both are made up of essentially free particles.

There is one important difference, however, illustrated in the movie at the top. In helium gas the particles are single helium atoms, but the particles in hydrogen gas are two atoms stuck together in an hydrogen molecule. Why the difference?