The lattice energy of NaCl has been determined experimentally to be −787 kJ mol⁻¹. Relevant ionic radii are:
Na⁺ 102 pm, Mg²⁺ 72 pm, Cl⁻ 181 pm, O²⁻ 140 pm.
Assume that, for a simple comparison, the magnitude of the lattice energy |U| is directly proportional to the product of the ionic charges divided by the cation–anion separation, and that the proportionality constant is the same for all four salts considered below.
Salts of interest:
(a) Using the proportionality stated, calculate the predicted ratio |U|(MgO)/|U|(NaCl).
(b) From the value of |U| for NaCl and the ratio obtained in part (a), estimate the lattice energy of MgO. Report both magnitude and sign.
(c) Briefly discuss two limitations of the simple proportionality model that could cause your estimate in part (b) to deviate from an experimentally measured value.