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3.4: Some Properties of Ionic Compounds

  • Page ID
    258844
  • Learning Objectives

    • To be introduced to basic properties of ionic compounds

    Ionic compounds form hard crystalline solids that melt at high temperatures and are resistant to evaporation. These properties stem from the characteristic internal structure of an ionic solid.

    Melting Points

    Because of the many simultaneous attractions between cations and anions that occur, ionic crystal lattices are very strong. The process of melting an ionic compound requires the addition of large amounts of energy in order to break all of the ionic bonds in the crystal. For example, sodium chloride has a melting temperature of about 800oC.

    Shattering

    Ionic compounds are generally hard, but brittle. It takes a large amount of mechanical force, such as striking a crystal with a hammer, to force one layer of ions to shift relative to its neighbor. However, when that happens, it brings ions of the same charge next to each other (Figure \(\PageIndex{1}\)). The repulsive forces between like-charged ions cause the crystal to shatter. When an ionic crystal breaks, it tends to do so along smooth planes because of the regular arrangement of the ions.

    Figure \(\PageIndex{1}\): (A) The sodium chloride crystal is shown in two dimensions. (B) When struck by a hammer, the negatively-charged chloride ions are forced near each other and the repulsive force causes the crystal to shatter.

    Conductivity

    Another characteristic property of ionic compounds is their electrical conductivity. Figure \(\PageIndex{2}\) shows three experiments in which two electrodes that are connected to a light bulb are placed in beakers containing three different substances.

    Figure \(\PageIndex{2}\): (A) Distilled water does not conduct electricity. (B) A solid ionic compound also does not conduct. (C) A water solution of an ionic compound conducts electricity well.

    In the first beaker, distilled water does not conduct a current because water is a molecular compound. In the second beaker, solid sodium chloride also does not conduct a current. Despite being ionic and thus composed of charged particles, the solid crystal lattice does not allow the ions to move between the electrodes. Mobile charged particles are required for the circuit to be complete and the light bulb to light up. In the third beaker, the \(\ce{NaCl}\) has been dissolved into the distilled water. Now the crystal lattice has been broken apart and the individual positive and negative ions can move. Cations move to one electrode, while anions move to the other, allowing electricity to flow (Figure \(\PageIndex{2}\)). Melting an ionic compound also frees the ions to conduct a current and ionic compounds conduct an electric current when melted or dissolved in water

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