Why does MgO have a higher melting point than CaS? They are both ionic compounds with the same charges, but CaS is heavier so shouldn't it have a higher melting point?
2 answers
The charge of a large ion is less concentrated because of its larger radius and surface area. That makes the attraction between Ca^2+ and S^2- ions weaker than the attraction between the smaller Mg^2+ and O^2- ions.
To check the which has higher melting point among the two ionic compounds, check the charge and ionic radius. The charge should be higher and size should be less for both cation and anion. So they will have higher ionic character and melting point