📘 IB Definition – Memorise Verbatim
"A metallic bond is the electrostatic attraction between a lattice of cations and delocalised electrons."
Mark-scoring checklist: You must say (1) electrostatic attraction, (2) cations (NOT atoms or nuclei), (3) delocalised electrons (NOT free electrons). Missing any one loses marks.
Metal atoms release their valence electrons into a communal "sea" of mobile electrons. These delocalised electrons are non-directional – they belong to no specific atom and move freely throughout the lattice. This model explains all of the characteristic physical properties:
| Property | Explanation Using the Model |
|---|---|
| Electrical conductivity | Delocalised electrons are free to move under an applied potential difference, carrying charge through the solid |
| Thermal conductivity | Delocalised electrons transfer kinetic energy rapidly through the lattice; cation vibrations also contribute |
| Malleability & Ductility | Layers of cations can slide over one another without disrupting the bonding – the electron sea simply redistributes around the new positions |
| High melting points | Strong electrostatic attraction between cations and the delocalised electron sea requires significant energy to overcome |
| Lustre (shine) | Delocalised electrons absorb and re-emit photons of visible light at many frequencies |
Factors Affecting Metallic Bond Strength
⬆️ Cation Charge
Higher charge → stronger electrostatic attraction → higher melting point. E.g. Mg²⁺ (3 e⁻ donated → actually 2) vs Na⁺ → Mg has a higher mp.
⬇️ Ionic Radius
Smaller cation → delocalised electrons are closer to the nucleus → stronger attraction. Na⁺ is larger than Mg²⁺, contributing to Mg's stronger bonding.
⚠️ Examiner Trap – "Free Electrons"
Never say "free electrons" – the correct term is "delocalised electrons". Also, metallic bonding is non-directional, which is why metals are malleable unlike ionic compounds (which shatter when layers shift).
🔑 Worked Comparison – Na vs Mg
Mg has a higher melting point (650 °C) than Na (98 °C) because: Mg²⁺ has a higher cation charge (+2 vs +1), contributes more delocalised electrons per atom (2 vs 1), and has a smaller ionic radius (72 pm vs 102 pm) – all resulting in a stronger electrostatic attraction.