Melting and boiling point trend in Group II

Question

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

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.

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.

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?

Answer 1

Beryllium ions are positively charged, small ions so they attract nuclear charge better than group 2 metals. This makes Beryllium crystal structure more covalent. So between layers, the bonds are stronger giving it a higher melting point and boiling point. This is the most common explanation - It can also be attributed to structure however nature of bond is the most common.

Unfortunately, Magnesium low melting point and boiling point is only attributed to its hexagonal close packed structure - There are generally no other consensuses for this however I can explain in detail Magnesium's structure makes it less tightly packed. It is arranged so there are arranged in layers with fewer neighboring atoms. Because there are fewer atoms, metallic bonds are weaker. Here is a good link

https://www.boyiprototyping.com/materials-guide/does-magnesium-have-a-low-melting-point/

Remember to overcome Melting point, you have to overcome the bonds in lattice and break lattice structure

There is no satisfactory explanations or trends at all in boiling points. Some people have tried to attributed it to the change in structure from Face-centered cubic to Body centered cubic at Barium and Radium but the boiling points are too variable. I will say to boil a metal, you have to overcome its metallic bonds and so enthalpy of atomization does become a key factor - not so much for melting points though as you have to break lattice.