What is an Ionic Bond?
An ionic bond is the electrostatic attraction between a positively charged ion (cation) and a negatively charged ion (anion). It forms when one atom donates electrons to another.
Why Does Electron Transfer Happen?
- Octet Rule: Atoms become stable with 8 electrons in their outermost shell (like noble gases).
- Metals (left of periodic table) have few valence electrons — far easier to lose them.
- Non-metals (right of periodic table) have many valence electrons — far easier to gain 1–2.
- This makes transfer energetically favourable for both atoms.
Comparing the 6 Compounds
- NaCl — Common salt. 1:1 ratio (Na⁺:Cl⁻). Lattice energy −787 kJ/mol.
- MgO — Magnesium oxide. 2+ and 2− charges give a MUCH stronger bond (−3791 kJ/mol lattice energy). Melts at 2852°C!
- CaF₂ — Calcium fluoride (fluorite). Ca²⁺ gives away 2 electrons to two F⁻ ions. 1:2 ratio.
- KBr — Potassium bromide. Similar to NaCl but larger ions → lower lattice energy.
- LiF — Lithium fluoride. Tiny ions → highest lattice energy of simple 1:1 salts (−1037 kJ/mol).
- Al₂O₃ — Aluminium oxide (corundum). Al³⁺ ion, 3 electrons transferred! Extremely high melting point (2072°C).
Crystal Lattice Structures
Ionic compounds don't exist as single molecules — instead billions of ions pack into a regular 3D lattice. Different compounds form different structures based on ion size ratio and charge.
Properties Explained by Ionic Bonding
- 🌡️ High melting point: Strong electrostatic forces require lots of energy to break.
- 💧 Dissolves in water: Polar water molecules attract and separate the ions (hydration).
- ⚡ Conducts electricity when dissolved/melted: Free-moving ions carry charge.
- 🧱 Brittle: Shifting layers aligns like charges, causing repulsion and cracking.