The method is based on finding a matrix relation between the ionic radii (the lattice energy U) and melting tem-
perature of ionic crystals of the MX type, where M is a Group 1A metal, and X is a halogen. From the equation
for the lattice energy U, a new equation has been derived for calculation of the melting temperature of ionic
crystals with the use of only the ionic radii and the degree of bond ionicity
: Tm = f(U, ). The average error of determination of Tm for alkali-metal halides is 2.80%. The melting temperatures of francium halides and alkali-
metal astatides (including FrAt) have been calculated. It has been shown that the accuracy of calculation of the
melting temperature of ionic crystals depends on the degree of bond ionicity: the error increases with an
increase in the covalent contribution. On the basis of the melting temperatures of metal halide crystals, a method
has been developed for the calculation of the melting temperatures of corresponding metals. The melting tem-
perature of francium has been calculated to be 24.861 ± 0.517C.