Ionic Bonding

The giant ionic lattice

• In the solid state, the cations and anions arrange themselves into a regular, repeating three-dimensional pattern with positive and negative ions alternating in every direction
• This is called a giant ionic lattice
• Each ion is surrounded by ions of the opposite charge as its nearest neighbours
• The whole solid is one continuous structure containing many millions of alternating ions

Ionic bonding as electrostatic attraction

• An ionic bond is the electrostatic attraction between oppositely charged ions
• The forces act in all directions and extend throughout the entire lattice
• These forces are very strong

High melting and boiling points

• Ionic compounds have high melting and boiling points
• Reason: melting or boiling the solid requires overcoming the many strong electrostatic forces between every pair of opposite ions in the lattice. That takes a lot of thermal energy
• The greater the charges on the ions, the stronger the attraction and the higher the melting point
  • Magnesium oxide (Mg²⁺ and O²⁻, both 2-charge) melts at about 2852 °C
  • Sodium chloride (Na⁺ and Cl⁻, both 1-charge) melts at about 801 °C

Electrical conductivity

• An electric current is a flow of charged particles
• In an ionic compound the ions are the charge carriers (not free electrons)
• Solid ionic compound: poor conductor. The ions are locked in fixed positions in the lattice and cannot move
• Molten or dissolved in water (aqueous): good conductor. The ions are free to move and carry charge

Solubility in water

• Many ionic compounds are soluble in water
• Water's polar molecules pull the ions out of the lattice and surround them in solution
• e.g. NaCl, KNO₃ and Na₂SO₄ all dissolve readily in water

Brittleness

• Ionic crystals are brittle
• A sharp blow can shift one layer of ions sideways across the next, bringing ions of the same charge next to one another. They repel, and the crystal cracks along that plane