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The following picture shows the melting and boiling point trends down group II elements.

Melting and boiling points of group II elements, done with Excel 2016

I have added question marks where the variability in data was rather disturbing (over two hundred degrees Celsius), or where the value is simply disputed. Nevertheless, this topic is not about the different results from measurements. Instead, it focuses on the "trend" in general.

For group I, we would say

  • the number of electron shells increases,
  • the increasing number of protons is unable to compensate for this fact,
  • therefore, atomic radius increases and
  • electronegativity, ionisation energies decrease,
  • hence the metallic bond is weaker $\Rightarrow$ melting and boiling points decrease
  • atomisation energy decreases.

Here are the atomic radii and ionisation energies of group II elements.

The atomic radii and ionisation energies of group II elements, done with Excel 2016

These (apart from radium) do seem to follow the same logic as in group I, however they do not correlate with the melting or boiling points. Some textbooks claim,

A good measure for the strength of a metallic bond is its atomisation energy.

Let us have a look.

Atomisation energies of group II elements, done with Excel 2016

Sure, this does agree better with the boiling and melting points. But note that it is actually not an explanation! Why do the atomisation energies behave in such a way?

This is where most resources either

  1. ignore the problem altogether by not mentioning it,
  2. give an explanation which is claimed by others to be faulty,
  3. few acknowledge the issue and say no simple explanation is possible.

The faulty explanation

Beryllium and magnesium have crystal lattices that are hexagonal closest-packed (close-packed?). Therefore, they have their own trend. Then there are calcium and strontium which have face-centered cubic lattices. Finally, the pair barium and radium have body-centered cubic structures.

Others say this is not enough, because the differences should not be as significant, especially for magnesium but also for barium. Barium has a 8-coordinated structure but has a higher boiling point than 12-coordinated strontium!

It might also be stated that melting points themselves are not the best estimator of the strength of bonds. The bonds are not all broken, but rather rearranged and "made weaker" as a result.

The question

As of now, is there an explanation for melting and boiling point trends of group II elements? Is there consensus?

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    $\begingroup$ This paper seems to have a quantitative explanation journals.aps.org/prb/abstract/10.1103/PhysRevB.62.14818 but I don't really understand the paper from a brief skimming. $\endgroup$
    – DavePhD
    Commented Mar 20, 2017 at 15:02
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    $\begingroup$ My 50 point bounty expired with no acceptable answer. Too bad, but it was worth the try. $\endgroup$
    – Ed V
    Commented Feb 16, 2020 at 15:02
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    $\begingroup$ @EdV I am inactive on Chem, but the bounty was appreciated, thank you. Indeed worth the try. $\endgroup$
    – Linear Christmas
    Commented Feb 16, 2020 at 15:07
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    $\begingroup$ There can be no rigorous answer to such questions I think. We have no more than six data points, and each of them is just the way it is. The reason for the melting points is of course the crystal structure, and the bond properties therein. It can be nothing else, but of course that doesn´t explain much. $\endgroup$
    – Karl
    Commented Feb 27, 2020 at 19:13
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    $\begingroup$ This question is similar to: Why does magnesium have an exceptionally low melting point?. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. $\endgroup$
    – Paul Kolk
    Commented Jul 8 at 12:17

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