Geologists have a rule of thumb: when molten rock cools and solidifies, crystals of compounds with the smallest lattice energies appear at the bottom of the mass. Suggest a reason for this.

Intermolecular forces are the interaction which are formed by the attraction of the two having opposite charges(partial positive and partial negative charge). The opposite charge are formed by the presence of the electron-negative atom in the molecule. Due to the presence of an electron-negative atom, there is an induced partial positive charge is generated on the electron-positive charge (or less electron-negative charge) atom. There will be an attraction between the both oppositely charges to form a bond.

Lattice energy can be defined as the energy required for the formation of the ionic crystal by the interaction of ions forming an ionic bond.

In these molten rock cools and solidifies but the process of solidification is different because of the difference in the lattice energy. When the molten rock is stable which has high lattice energy which solidifies from the top layer but when the molten rock is unstable is solidifies from the bottom first. The unstable molten rock has low lattice energy.