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Compare the melting points of $\ce{BeF2, MgF2, CaF2, SrF2, BaF2}$.

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Actual order is $\ce{ BeF2 > CaF2 > SrF2 > BaF2 > MgF2}$.

I have worked out the following things while comparing their melting points.

a) Melting points of ionic compound increases as the ionic interactions between cation and anion in the metal lattice increase. In this case, smaller cations will result in stronger interactions. That gives this order : $\ce{ BeF2 > MgF2 > CaF2 > SrF2 > BaF2}$.

b) None of these compounds is a covalent solid. If it were, its melting point would have been higher.

c)The order of melting point also depends upon the crystal lattice structure.

(i) Be and Mg crystallise in hcp, Ca and Sr in fcc and Ba in bcc.

(ii) The cations will occupy the lattice points and since fluoride ions are smaller than cations, they will occupy the voids.

(iii) The more is the coordination number, the more will be repulsions between similar ions. In the lattice, when two cations are in contact, there will be repulsions. But, the repulsions will be less when the size of carion will be small since in that case, the electron cloud of one cation will experience nuclear attraction by the other cation.

(iv) Hcp and fcc have the same coordination number.

(v) Based on this logic, the order should be: $\ce{ SrF2 > CaF2 > MgF2 > BeF2 }$.

(vi) Although, the coordination number of Ba is very less, it size is very big. So, we cannot say with certainty if it would have the lowest melting point.

Now, I don't know how to reconcile these two points, the factors are totally opposing. Where to put Barium? This made me think that the order cannot be predicted with certainty just based upon theoretical analysis of their lattice structures.

It is important for me to know whether the points that I have mentioned correct or not. Are there more factors that could influence their melting points? And finally, if the order can be predicted with certainty purely on theoretical basis, how would that be done?

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  • $\begingroup$ So are you asking about the elements or are you asking about the fluorides? $\endgroup$ – orthocresol Mar 5 '17 at 8:15
  • $\begingroup$ The fluorides...I understand that the fluorine ions will just occupy the voids made by the metal lattice,..so how does it matter if we analyze the crystal structure of the metal lattice or the metal fluorides. Correct me if I am wrong $\endgroup$ – Arishta Mar 5 '17 at 8:32
  • $\begingroup$ related chemistry.stackexchange.com/questions/53447/… $\endgroup$ – Mithoron Mar 5 '17 at 16:39
  • $\begingroup$ Your assumption that "that the fluorine ions will just occupy the voids made by the metal lattice" seems not to be correct to me. $\endgroup$ – aventurin Mar 5 '17 at 18:58
  • $\begingroup$ @aventurin Correct me. $\endgroup$ – Arishta Mar 6 '17 at 6:46

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