STRUCTURES AND INTERMOLECULAR FORCES

• Intermolecular forces are attractive or repulsive forces between molecules, caused by partial charges. The attractive forces are the ones that work to overcome the randomizing forces of kinetic energy.

• Ion–Dipole Intermolecular Forces, These forces are due to the attraction of an ion and one end of a polar molecule (dipole).

• Dipole–Dipole Intermolecular Forces, These forces result from the attraction of the positive end of one dipole to the negative end of another dipole.

• Hydrogen bonding is a special type of dipole–dipole attraction in which a hydrogen atom is polar-covalently bonded to one of the following extremely electronegative elements: N,O, or F.

• Ion-Induced Dipole and Dipole-Induced Dipole Intermolecular Forces ,These types of attraction occur when the charge on an ion or a dipole distorts the electron cloud of a nonpolar molecule and induces a temporary dipole in the nonpolar molecule.

• London (Dispersion) Intermolecular Force , This intermolecular attraction occurs in all substances, but is significant only when the other types of intermolecular forces are absent. It arises from a momentary distortion of the electron cloud, with the creation of a very weak dipole.

THE LIQUID STATE

• The amount of force required to break through this molecular layer at the surface is called the liquid’s surface tension.

• Viscosity, the resistance of liquids to flow, is affected by intermolecular forces, temperature, and molecular shape.

• Capillary action is the spontaneous rising of a liquid through a narrow tube, against the force of gravity. It is caused by competition between the intermolecular forces in the liquid and those attractive forces between the liquid and the tube wall.

• high heat capacity, the heat absorbed to cause the temperature to rise, and a high heat of vaporization, the heat needed to transform the liquid into a gas.

THE SOLID STATE

• Solid is defined as a substance that has both a definite volume and a definite shape.

• Amorphous solids lack extensive ordering of the particles.

• Crystalline solids display a very regular ordering of the particles in a three-dimensional structure called the crystal lattice. In this crystal lattice there are repeating units called unit cells.

• Five types of crystalline solid are known:

  1. In atomic solids, individual atoms are held in place by London forces. The noble gases are the only atomic solids known to form.

  2. In molecular solids, lattices composed of molecules are held in place by London forces, dipole–dipole forces, and hydrogen bonding. Solid methane and water are examples of molecular solids.

  3. In ionic solids, lattices composed of ions are held together by the attraction of the opposite charges of the ions. These crystalline solids tend to be strong, with high melting points because of the strength of the intermolecular forces. NaCl and other salts are examples of ionic solids. Figure 12.3 shows the lattice structure of NaCl. Each sodium cation is surrounded by six chloride anions, and each chloride anion is surrounded by six sodium cations.

  4. In metallic solids, metal atoms occupying the crystal lattice are held together by metallic bonding. In metallic bonding, the electrons of the atoms are delocalized and are free to move throughout the entire solid. This explains electrical and thermal conductivity, as well as many other properties of metals.

  5. In covalent network solids, covalent bonds join atoms together in the crystal lattice, which is quite large. Graphite, diamond, and silicon dioxide (SiO2) are examples of network solids. The crystal is one giant molecule.