This is the Higher Level lesson. The core S2.3.1–S2.3.2 (electron-sea model, properties, bond strength) is shared with SL; the HL-only content — the transition elements (S2.3.3) — carries a purple HL badge.
Core — SL & HL
HL AHL only
👆 switch on the current · compare the metals
1. The electron-sea model S2.3.1
In a metal, each atom loses its outer electrons to become a positive cation. Those electrons are no longer attached to any one atom — they are delocalised, free to move through the whole structure as a "sea". The metallic bond is the electrostatic attraction between the fixed lattice of cations and this sea of delocalised electrons. Press Apply voltage and watch the free electrons drift — that flow is an electric current.
Animated electron-sea model
Random: the delocalised electrons move in all directions between the fixed metal cations.
2. Why metals behave as they do S2.3.1
Every classic property of a metal comes straight from the electron-sea model. Click each:
3. How strong is the metallic bond? S2.3.2
The strength of metallic bonding — and so the melting point — increases with the charge of the cation, a smaller ionic radius, and more delocalised electrons per atom. Compare three period-3 metals:
Melting points of period-3 metals
Click a barNa → Mg → Al: the cation charge rises 1+ → 2+ → 3+ and each atom releases more delocalised electrons, so the metallic bond gets stronger and the melting point rises.
4. The transition elements HL
Transition elements bond even more strongly than the s- and p-block metals. As well as their outer s electrons, they can delocalise some of their d-electrons into the sea. More electrons in the sea, plus small ions, means a much stronger metallic bond — which is why transition metals have very high melting points and excellent electrical conductivity. Compare them with the period-3 metals:
Melting points: s/p-block vs transition metals · click a bar
Click a barSodium and aluminium (s/p block) melt far below the transition metals. Iron and tungsten delocalise d-electrons too, giving many more electrons in the sea and much stronger bonding — tungsten melts at 3422 °C, the highest of any metal.
Common mistakes examiners see
What is a metallic bond?✗ The sharing of electrons between two metal atoms.✓ The electrostatic attraction between a lattice of positive cations and a sea of delocalised electrons.
Why can metals be bent and shaped (malleable)?✗ The bonds break and reform as new bonds.✓ Layers of cations slide over one another while the delocalised electrons keep holding everything together, so the metal deforms without shattering.
Why does aluminium have a higher melting point than sodium?✗ Aluminium atoms are heavier.✓ Al³⁺ has a higher charge, a smaller radius and 3 delocalised electrons per atom (vs 1 for Na⁺), so the metallic bond is much stronger.
Why do metals conduct electricity?✗ The cations move and carry charge.✓ The delocalised electrons are free to move and carry the charge; the cations stay fixed in the lattice.
Why do transition metals have especially high melting points?✗ They have bigger atoms.✓ They delocalise d-electrons as well as s-electrons, so there are more electrons in the sea and a stronger metallic bond. HL