Question

Which of the following compounds has the lowest melting point and which has the highest melting point? Why? AlF3 RbF NaF MgF2 CaF2

Answer 1

Ionic compounds with higher charges have higher boiling points because there are very strong intermolecular forces which are caused by the huge charges (+3 in Al and -3 in F3).

Lower charges have lower melting points because less energy is needed to overcome this attraction.

The other RbF and NaF have the same charges (+1 and -1) but different atomic radii on the cations. Na is smaller, so it has the most attraction because of the small distance to the negatively charges nucleus. Rb is larger,. Larger size decreases boiling point.

However, charge affects boiling point more than size, so always look at charge first, then break ties in ranking by looking at atomic size.

So teh compound with highest MP is AlF3 and with lowest Mp is NaF.

Thank you. Hope this helped.