Question

Consider that the following common ions have the given radii (in parenthesis): lithium ion (134 pm), magnesium ion (130 pm), fluoride ion (119 pm), and sulfide ion (170 pm). Based on this information, which compound would you expect to have the larger lattice energy lithium fluoride or magnesium sulfide? Why?

Answer 1

lattice energy is given by

U = - k Q1 Q2/r₀

where,

r_{0 is the internuclear distance}

Q1 and Q2 are charges of the ions

_{lattice energy is inversly proportinal to internuclear distance}

_{for LiF inter neuclear distance = 134pm + 119pm = 253pm}

_{for MgS inter neuclear distance = 130pm + 170pm = 300pm}

_{so, cosidering ionic radius only Lithium floride is should have the larger lattice energy}

_{cosidering overall magnesium Sulphide should have the larger lattice energy because both magnesium and sulphide ions are bivalent but both Lithium and Flourine are monovalent}

_{lattice energy of LiF = -1050kJ/mol}

_{Lattice energy of MgS = -3406kJ/mol}