Intermolecular Forces

Bonding in Solids

• Consist of atoms or molecules held together by intermolecular forces (dipole-dipole, dispersion and hydrogen bonds)
• These forces are weaker than chemical (covalent) bonds. Therefore molecular solids are soft, and have a generally low melting temperature
• Most substances that are gasses or liquids at room temperature form molecular solids at low temperature (e.g. H₂O, CO₂)

Comparison of bond energies for different types of bonds

• The properties of molecular solids also depends upon the shape of the molecule

• Benzene (six carbon ring with a symmetrical structure) packs efficiently in three dimensions
• Toluene is related to benzene but has a methyl group attached to one carbon of the ring. It is not symmetrical and does not pack efficiently. Its melting point is lower than benzene

• Consist of networks or chains of molecules held together by covalent bonds
• Covalent bonds are stronger than intermolecular forces and covalent substances are subsequently harder and have higher melting temperatures
• Diamond is a covalent structure of carbon. It is extremely hard and has a melting temperature of 3550°C

• Held together by ionic bonds
• The strength of the ionic interactions depends on the magnitude of the charge of the ions. Thus, NaCl (single charge on both ions) has a melting point of 801°C, whereas MgO (2⁺, 2^- charge on the ions) has a melting point of 2852°C

• Consist entirely of metal atoms.
• Typically hexagonal close packed, cubic close packed or body-centered cubic structures. These have coordination numbers of either 12 or 8.
• Bonding is due to valence electrons which are delocalized throughout the entire solid
• Bonding is stronger than simple dispersion forces, but there are insufficient electrons to form ordinary covalent bonds. The strength of the bonding increases with the number of electrons available for bonding
• Delocalization of electrons is the physical basis for the ability of metals to carry electrical current (electrons are free to move about the metal structure)
• The nucleus and inner core of electrons are in a "sea" of delocalized, mobile valence electrons

1996 Michael Blaber