Q.Which would you expect to have a higher melting point, magnesium oxide or magnesium fluoride? Explain your reasoning.

Step 1. Melting point of an ionic solid tracks its lattice enthalpy -- the strength of the electrostatic (Coulombic) attraction holding the cation and anion lattice together, which is proportional to the PRODUCT of the ionic charges and inversely related to the distance between them.

Step 2. Mg²⁺ is the common cation in both compounds, so the difference between MgO and MgF₂ comes down to the anion: O²⁻ carries a charge of -2, while F⁻ carries a charge of only -1.

Step 3. Since lattice energy scales with the product of the ionic charges (q⁺ × q⁻), and the two ions are of broadly comparable size, the doubly-charged O²⁻ in MgO gives roughly double the Coulombic attraction (and hence much greater lattice enthalpy) compared to the singly-charged F⁻ in MgF₂. …

Compare the two anions' charges (O2- vs F-) at a similar ionic size, and apply the rule that lattice energy (and hence meltin …

• Trying to explain the melting-point difference by ionic size alone -- O2- and F- are close enough in size that the real dri …