Gases are not the only form of matter in the universe. Liquids and
solids also exist, especially with larger molecules and at lower
temperatures. Every atom or molecule has a weak attraction for other
atoms and molecules, or a "stickiness" on contact, known as van
der Waals attraction. If the temperature is low and the energy of
motion of a collection of molecules is small enough, this van der
Waals attraction will hold the molecules together in a liquid.
The molecules remain in contact but are free to slide past one another.
At even lower temperatures and molecular energies, this freedom
of motion is reduced further, and the molecules become locked into
the frozen geometry of a solid.
Tiny particles such as He and
must be cooled to extremely low temperatures before they condense
to a liquid or freeze to a solid. Larger molecules with more surface
area have greater van der Waals "stickiness," and occur as liquids
or solids at room temperature.
Some atoms can gain or lose electrons to become electrically charged
ions. These ions are held together in solids known as salts
by the electrostatic forces between ions of opposite charge. After
the study of bond-making-and-breaking reactions that molecules can
undergo, one of the most important areas of chemistry is to explain
the behavior and properties of substances in terms of the interactions
between the molecules of which they are made.