Metallic bonding gets quantified, and d-electrons appear

A lattice of positive ions in a "sea of delocalised electrons", explaining conductivity and malleability.

The same picture, but strength is now explained by charge on the cation and ionic radius (Structure 2.3.2) — Mg is harder and higher-melting than Na because Mg²⁺ contributes two delocalised electrons and is smaller. Transition elements additionally delocalise d-electrons (2.3.3), which is why they are hard, dense and high-melting compared with group 1.

From a descriptive picture to a comparative, predictive one; you can now rank metals.

GCSE only needs the picture to explain conductivity.

Cannot explain why transition metals melt so much higher than alkali metals; cannot compare Na/Mg/Al.