3) Ionic Compounds

Lattice Structure of Ionic Compounds

  1. Ionic compounds have a structure called a giant ionic lattice.

  2. The ions are arranged in a closely packed, regular lattice.

  3. Electrostatic attractions between oppositely charged ions hold the lattice together, forming ionic bonds.

Representing the Lattice

  1. The lattice can be shown in various models:

    • Ball and stick model: Shows the regular pattern of an ionic crystal and the relative sizes of ions, but only displays the outer layer of the compound.

    • Diagram model: Represents the alternating pattern of Na⁺ and Cl⁻ ions (for sodium chloride), but isn’t to scale and has gaps that don’t exist in reality.

  2. Sodium chloride (NaCl): Learn its structure well, where Na⁺ and Cl⁻ ions alternate.

Properties of Ionic Compounds

  1. High melting and boiling points: Due to the strong ionic bonds between the ions, which require a lot of energy to overcome.

  2. Conductivity:

    • Solid ionic compounds don’t conduct electricity because the ions are held in place.

    • When melted or dissolved in water, the ions are free to move and can carry an electric charge.

  3. Solubility: Many ionic compounds dissolve in water, allowing the ions to move freely and conduct electricity in the solution.

Finding the Empirical Formula

  1. You may need to work out the empirical formula of an ionic compound from a diagram.

  2. If using a dot and cross diagram:

    • Count how many atoms there are of each element and write this down to give the empirical formula.

  3. If using a 3D diagram of the lattice, identify the ions present in the compound and balance the charges so that the overall charge is zero.

Example: Potassium Oxide (K₂O)

  1. Potassium (K) is in Group 1, so it forms 1⁺ ions.

  2. Oxygen (O) is in Group 6, so it forms 2⁻ ions.

  3. To balance the charges, two K⁺ ions are needed to balance the 2⁻ charge of the oxide ion, resulting in the empirical formula K₂O.