Four main types of interatomic force are found in solids. Ionic bonds occur in inorganic crystal lattices such as salts. In sodium chloride, for example, sodium cations and chloride anions are arranged alternately in a rigid, uniform structure. Metallic bonding occurs when atoms give up their outermost electrons to form a "sea." The positively charged metal ions are held in a lattice in this sea. Covalent bonds occur in organic compounds and inorganic silicates and are formed by adjacent atoms contributing one electron or more to form a bond. Van der Waals forces are induced when the nucleus of an atom in one molecule comes into proximity to the electrons of an atom in another molecule. These are the weakest type of interatomic force; however, they still influence the melting point of a solid and the boiling point of a liquid.
Below a certain temperature atoms are held together in rigid or semirigid structures by one or more interatomic force. However, at a certain temperature the structures break down to form a less ordered phase called a liquid. This temperature is called the melting temperature. As crystalline and polycrystalline lattices are bound together by strong electrostatic forces, a large amount of energy is needed to break these lattices. This energy is called lattice energy. The lattice energies of sodium chloride and magnesium oxide are very high, so their respective melting points are 1486 degrees F and 5252 degrees F. The bonding forces in metal lattices are also very strong. Lead melts at 622 degrees F, aluminum at 1220 degrees F and iron at 2804 degrees F.
Compared with ionic or metallic structures, covalently-bonded structures are relatively small, the forces between their molecules are much weaker and they have much lower melting points. At room temperatures they may be vapors, liquids or solids. Covalent compounds are usually held in solid form by weak van der Waals forces; however, some molecules experience another type of interatomic attraction called hydrogen bonding.
Hydrogen bonding occurs between the hydrogen atoms on one molecule and the lone, otherwise nonbonding electron pairs on an adjacent molecule. Like van der Waals forces, hydrogen bonds are comparatively weak; however, they are strong enough to provide structure for certain types of molecule and affect the phase transition temperatures of others. For example, hydrogen bonding holds together the two strands of the DNA double helix. Similarly, hydrogen bonding occurs between hydrogen and oxygen in liquid water molecules. Without hydrogen bonding DNA would unravel and break down, and the boiling point of water would be much lower, so this subtle force has been essential in the development of life.