In practice, before this point is reached, van der Waals attractions and molecular volumes become too important to be ignored, and gases deviate from ideal behavior. The most striking deviation occurs when slowly moving molecules "stick" to one another, and a gas condenses into a liquid. At still lower temperatures, the liquid freezes into a crystalline solid.

The boiling point of a liquid is a useful measure of the strength of van der Waals forces between molecules, because the smaller the molecules and the weaker these forces are, the lower the temperature can be before the gas molecules stick together and condense as a liquid. Of the two elements in our simple universe, molecules must be cooled to -253, or 20K, before they condense. This is the boiling point of liquid hydrogen at a pressure of
1 atm.

The single atoms of helium gas are smaller, with less surface area. They must be cooled to 4K before their attractive forces cause them to condense.

Hydrogen and helium illustrate many chemical properties, but by themselves they are a dead end. They are not capable of the great variation seen in the chemistry of the heavier elements. If stellar syntheses had gone no farther than hydrogen fusion, the universe would have been stillborn. To continue, we must turn to the elements heavier than helium, and this is the subject of the next chapters.