Why does magnesium fluoride has a higher melting point than fluorine and carbon tetrafluoride?

Magnesium Flouride is made up of ionic bonds, so a high melting point.
Where as fluorine and Carbon Tetrachloride is made up of covalent bonds, so only weak bonds and low melting point.

Determine the  wavelength of light emitted when an electron  in an li2+ ion  makes a transition from an orbital with n  =  6 to an orbital with n  = 5.