Properties of Differently Bonded Substances

The properties of substances vary by how the particles in the substances are bonded to each other. In particular, the relative strengths of the bonds should be compared. In general, covalent bonding is the strongest type of bond followed by ionic, metallic, and then intermolecular bonding, which is the weakest. Hydrogen bonding is the strongest type of intermolecular bond followed by dipole-dipole attraction and then London dispersion forces (LDFs). These LDFs become stronger when more electrons are present. When laboratory experiments find, in general, the properties predicted by the various bonding models, credence is given to the theory describing the bonding.

Properties of Ionic Substances

Ionic substances are those in which fully charged ions are bound by ionic bonds. Laboratory experiments reveal that, in general, ionic substances are characterized by the following properties:

  1. In the solid phase at room temperature, they do not conduct appreciable electric current.
  2. In the liquid phase, they are relatively good conductors of electric current. 
  3. They have relatively high melting and boiling points. There is a wide variation in the properties of different ionic compounds. For example, potassium iodide (KI) melts at 686°C and boils at 1,330°C, while magnesium oxide (MgO) melts at 2,800°C and boils at 3,600°C. Both KI and MgO are ionic compounds.
  4. They have relatively low volatilities and low vapor pressures. In other words, they do not readily evaporate at room temperature.
  5. They are brittle and easily broken when stress is exerted on them.
  6. Those that are soluble in water form electrolytic solutions that are good conductors of electricity. There is, however, a wide range in the solubilities of ionic compounds. For example, at 25°C, 92 grams of sodium nitrate (NaNO3) dissolves in 100 grams of water, while only 0.0002 grams of barium sulfate (BaSO4) dissolve in the same mass of water.

Properties of Molecular Substances

Molecular substances include those in which molecules are bound by intermolecular forces of attraction (i.e., dipole-dipole, LDFs, or hydrogen bonding). Covalent bonds are present within the molecules (intramolecular) but do not dictate the properties of a molecular substance.

Experiments have shown that molecular substances have the following general properties:

  1. Neither the liquids nor the solids conduct electric current appreciably.
  2. Many exist as gases at room temperature and atmospheric pressure, and many of the solids and liquids are relatively volatile.
  3. The melting points of the solid crystals are relatively low.
  4. The boiling points of the liquids are relatively low.
  5. The solids are generally soft and have a waxy consistency.
  6. Polar molecular substances generally dissolve in water. Nonpolar molecular substances generally dissolve in nonpolar molecular solvents.
  7. A large amount of energy is often required to decompose the substance chemically into simpler substances.

Properties of Metallic Substances

Metallic substances include those in which atoms are bound to each other by metallic bonding. Experiments reveal that metallic substances have the following general properties:

  1. All metals, except for mercury, exist as solids at room temperature as they have relatively high melting points.
  2. Metals conduct electricity in both the solid and liquid states.
  3. Metals conduct heat well.
  4. Metals have low vapor pressures.
  5. Metals are not soluble in water.