Explain the difference in conductivity btwn magnesium and molten magnesium chloride

Magnesium conducts electricity due to the presence of delocalized electrons, which are free to move throughout its metallic lattice. Molten magnesium chloride, an ionic compound, conducts electricity because its ions ($\ce{Mg^2+}$ and $\ce{Cl^-}$) become mobile when heated to their melting point, allowing them to carry charge.