15.Ionic Compounds: Structure, Properties & Formula

1. Structure: Giant Ionic Lattices

  • Lattice Structure: In real life, ionic compounds aren't just single pairs of ions. They consist of millions of ions that alternate in a three-dimensional arrangement called a regular lattice structure.

  • Electrostatic Attraction: Each ion is attracted to all the oppositely charged ions surrounding it in every direction.

  • Representation: These can be shown as 3D models or "ball and stick" diagrams, where sticks represent the ionic bonds.

2. Physical Properties

  • High Melting and Boiling Points: * Ionic bonds are very strong.

    • Because there are so many of these bonds in a giant lattice, it requires a huge amount of energy to break them.

  • Electrical Conductivity:

    • As Solids: They cannot conduct electricity because the ions are fixed in place and cannot move.

    • When Melted (Molten) or Dissolved in Water (Aqueous): They can conduct electricity because the lattice breaks down, allowing the ions to move freely and carry a charge.

3. Determining the Formula

The goal when writing a formula is to ensure the total positive charges balance out the total negative charges to make the compound neutral.

  • Simple Ions:

    • Sodium Chloride: Na+ and Cl- balance perfectly as NaCl.

    • Magnesium Chloride: Mg is in Group 2 (Mg2+), so it needs two Cl- ions to balance, making it MgCl2.

  • Complex (Polyatomic) Ions: These are groups of atoms with an overall charge. You should memorize these:

    • Hydroxide: OH-

    • Sulfate: SO4 2-

    • Nitrate: NO3-

    • Carbonate: CO3 2-

    • Ammonium: NH4+

  • Using Brackets: If you need more than one polyatomic ion, put it in brackets.

    • Example: Calcium Hydroxide is Ca(OH)2 because you need two OH- ions to balance one Ca2+ ion.

  • Finding the Common Multiple: For Aluminum Sulfate (Al3+ and SO4 2-), the lowest common multiple is 6. You need two Al3+ and three SO4 2-, resulting in Al2(SO4)3.