S2.3 The metallic modelIB Chemistry HL: Revision notes
Section 1
The metallic bond
Metal atoms lose their outer electrons into a shared pool of delocalised electrons, leaving a regular lattice of cations. A metallic bond is the electrostatic attraction between a lattice of cations and delocalised electrons. The attraction acts in every direction, so metallic bonding is non-directional.
Section 2
Conductivity and malleability
Electrical conductivity: delocalised electrons are mobile and drift towards the positive terminal when a potential difference is applied — in the solid and the liquid.
Thermal conductivity: delocalised electrons carry kinetic energy rapidly through the lattice, and closely packed cations pass on vibrations.
Malleability and ductility: layers of cations slide over one another; the non-directional attraction to the delocalised electrons holds them in their new positions, so the metal bends rather than shatters.
The cations do not move through a metal wire — only the delocalised electrons carry the current.
Section 3
Strength of metallic bonding and melting points
Metallic bonding is stronger when the cation has a higher charge (more delocalised electrons per atom) and a smaller radius.
- Down a group (e.g. Li 181 °C → Cs 28 °C): same charge, larger radius → weaker bonding → lower melting point.
- Across period 3 (Na 98 °C, Mg 650 °C, Al 660 °C): higher charge, smaller radius, more delocalised electrons → higher melting point.
- Compare like with like: Li⁺ (76 pm) and Mg²⁺ (72 pm) are almost the same size, so magnesium's higher melting point is due to its 2+ charge; Be²⁺ and Mg²⁺ share a 2+ charge, so beryllium's higher melting point is due to its smaller radius.
When comparing two metals, identify which factor is the same and which differs — then argue from the one that differs.
Section 4
Transition elements have delocalised d-electrons (HL)
In transition elements the 3d and 4s sublevels are very close in energy, so d-electrons are delocalised as well as s-electrons. Iron ([Ar]4s²3d⁶) and titanium ([Ar]3d²4s²) therefore have many more delocalised electrons per atom than calcium ([Ar]4s²), which delocalises only its two 4s electrons.
More delocalised electrons means stronger electrostatic attraction to the small cations, giving high melting points (Sc–Ni: 1246–1910 °C, compared with K 63 °C and Ca 842 °C) and good electrical conductivity, because there are many mobile charge carriers.
Section 5
Where the simple model runs out (HL)
Zinc ([Ar]3d¹⁰4s²) melts at only 420 °C. Its full 3d sublevel holds its d-electrons tightly, so only the two 4s electrons are delocalised — much less than in nickel, whose partly filled 3d sublevel contributes to the electron sea.
But Zn²⁺ (74 pm) is smaller than Ca²⁺ (100 pm), so the charge-and-radius model alone would predict zinc melts above calcium (842 °C). It does not, which shows that lattice structure and packing also affect melting point. Evaluating a model's limits like this is a typical AO3 task.
Must know
- Metallic bond: attraction between a lattice of cations and delocalised electrons; non-directional.
- Delocalised electrons → electrical and thermal conductivity; sliding layers → malleability.
- Higher charge and smaller radius → stronger bonding → higher melting point.
- (HL) Transition elements delocalise d-electrons as well as s-electrons → high melting points and good conductivity.
- (HL) Zinc's full 3d sublevel contributes no delocalised d-electrons, so it melts at a low temperature.
That's the notes covered.
Carry on to the next subtopic.