The density and melting point of a metal, taken together, can give a useful qualitative guide to the chemical reactivity of the metal. A high density and high melting point indicate a low reactivity; conversely, low density and low melting point indicate high reactivity. The melting point is allied to the heat of sublimation; the density gives a guide to the magnitude of the ionization energy. These two thermodynamic energy quantities are common to the Born-Haber cycles for the standard electrode potential, enthalpy of formation of the oxide, and reaction with water, which are commonly accepted thermodynamic measures of reactivity. Kinetic effects also play a role, not easy to quantify. A sharp discontinuity in the general trend of either melting point or density in the middle of a long period points to anomalous or unusual chemical reactivity of the metal.
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