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.
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
molecule. Why the difference?