Explain why metals have high thermal conductivity and why this property decreases when the metal forms an alloy.

In a metal the valence electrons are delocalised and move freely through the lattice. When the metal is heated these electrons gain kinetic energy and carry it rapidly from the hot to the cold side, giving the metal a high thermal conductivity.

When the metal is alloyed, atoms of a different element (usually of a different size) are introduced into the lattice. This distorts the regular arrangement of atoms and creates additional scattering sites for the delocalised electrons. The increased electron scattering reduces the mean free path of the electrons, so they transfer heat less efficiently and the alloy’s thermal conductivity falls.