If two hydrogen atoms are brought together at a moderate temperature, they will bind to one another to form an H-H or molecule.

Helium atoms do not behave in this way. When they collide, they bounce away unchanged and show little tendency to associate.

The concept of the chemical bond that holds H atoms together, but not those of He, is the most important single idea in chemistry.

When do bonds form between atoms, and why, and in what directions? How do these bonds determine how the resulting chemical substances behave?

At temperatures similar to those on our planet, helium atoms (He) and hydrogen molecules () move about individually. Each atom or molecule in a gas moves independently with a speed that depends on its energy of motion. The higher the temperature, the faster the atoms or molecules of a gas move; and temperature in fact is a direct measure of the average energy of the molecules of a gas.