📘 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).
Explaining Metallic Properties
Metals are excellent electrical conductors because their delocalised electrons carry charge when a potential difference is applied. They are also ductile (can be drawn into wires) and malleable (can be hammered into sheets) because the layers of cations slide over each other without breaking the metallic bond. The non-directional electron sea simply adjusts to the new arrangement.
This contrasts with ionic compounds, which shatter when layers are displaced because ions of the same charge are forced next to each other, creating repulsive forces.
Practical Applications
Copper is used for electrical wiring because it combines high electrical conductivity with excellent ductility. Aluminium is widely used in aircraft construction due to its low density, good strength-to-weight ratio, and resistance to corrosion. Iron is used in construction as steel due to the strength of its metallic bonding.
📐 Worked Example: Predict the Melting Point Order of Na, Mg, and Al
Sodium (Na): Forms Na⁺ cations with charge +1, contributes 1 delocalised electron per atom, and has a relatively large ionic radius (102 pm). This gives the weakest metallic bond of the three.
Magnesium (Mg): Forms Mg²⁺ with charge +2, contributes 2 delocalised electrons per atom, and has a smaller ionic radius (72 pm). Stronger electrostatic attraction than Na.
Aluminium (Al): Forms Al³⁺ with charge +3, contributes 3 delocalised electrons per atom, and has the smallest ionic radius (53 pm). This creates the strongest metallic bond.
Predicted order: Na (98 °C) < Mg (650 °C) < Al (660 °C) ✓
Note: Mg and Al have similar melting points despite Al's stronger bond because the crystal structure also plays a role. The IB expects you to focus on charge and radius.
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