Why does the melting point of group 2 oxides decrease down the group?

Answer 1

Group Two elements form oxide consisting of ionic bonds. With that in mind, as you go down group two, the ionic radius increases due to an addtion of a electron shell, so therefore, melting point of group2 oxide decreases down group because the distance between the O2- and Group two ion are further apart. The increase of bond length results in less energy needed to break the ionic bonds, resulting in decrease of energy.

Answer 2

The solubility of a substance is determined by the strength of the reverse lattice energy(opposite of lattice energy) and the hydration energy. A substance is soluble when the reverse lattice energy is greater than the hydration energy.

Moving down group 2 the atomic radii increases( the repulsive force is greater than the attractive force of the nuclear charge). Unlike sulphates or carbonates the size of the hydroxide ion is small and so the sum of the anion and cation would be large leading to a decrease in lattice energy since the two are inversely proportional. Because of this the hydration energy outweighs the lattice energy and so the solubility of the hydroxides increases down the group.

Answer 3

The melting point of group 2 oxides decreases down the group because the atomic size of the cations (Group 2 metals) increases, leading to weaker metallic bonding with the oxide ions. This results in lower energy required to overcome these weaker bonds, hence lowering the melting point.