10
$\begingroup$

Why does magnesium have a lower melting point than both calcium and beryllium? It does not seem to fit into the group trend.

$\endgroup$
  • 1
    $\begingroup$ Melting points are a little more complicated. I suspect this mainly lies in the magnesium's crystal structure. $\endgroup$ – M.A.R. Jun 9 '15 at 12:39
  • 2
    $\begingroup$ I'm not sure I'd consider 922 K (Mg) relative to 1112 K (Ca) exceptionally low. And given Sr (1041 K), Ba (1002 K) and Ra (973 K) perhaps the question might be why is calcium's melting point a tad higher than you might expect. Now if magnesium's melting point were 250 K, that might be exceptionally low compared with the rest of the column. $\endgroup$ – Jon Custer Jun 9 '15 at 14:42
  • $\begingroup$ @Jon that's my fault. You can edit the question accordingly and apply better wording. $\endgroup$ – M.A.R. Jun 9 '15 at 16:06
  • $\begingroup$ @M.A.Ramezani - no blame is implied! I'm just not sure what better wording would be - I just don't see a huge difference in the melting points. The crystal structure plays some role - by the SGTE database, hcp Ca would melt at about 940 K, so pretty similar to Mg. But, that just pushes the question further down to why the stable crystal structure at room temperature is changing from hcp to fcc to bcc going down the row (and why Ca changes from fcc to bcc before melting). $\endgroup$ – Jon Custer Jun 9 '15 at 16:35
  • $\begingroup$ Related: Why does magnesium have the lowest melting point of all earth alkalis? $\endgroup$ – CowperKettle May 9 '16 at 5:57
4
$\begingroup$

Following up on my comment, even though I'm not convinced that this is a 'real' answer to the question. Consulting the SGTE database of elemental free energies (AT Dinsdale, CALPHAD 15(4) 317-425 (1991) with various updates over the years), one can obtain the melting temperatures for the hcp, fcc, and bcc phases of Be, Mg, Ca, Sr, and Ba. (Rough guesses looking at plots - consider good to ~5K):

Element hcp bcc fcc melting points (K), phases at RT and melt:

Be 1544, 1560, 1018, hcp at RT, bcc at melt

Mg 922, 755, 705, hcp at RT, hcp at melt

Ca 940, 1114, 1063, fcc at RT, bcc at melt

Sr 900, 1050, 1030, fcc at RT, bcc at melt

Ba 635, 1000, 709, bcc at RT and at melt

Several things to note, amongst them the variety of which crystal structure is most stable for the different elements at different temperatures. Also, the hcp phase's melting point does almost(!) monotonically decrease going down the table. The biggest anomaly would actually seem to be the relative instability of the bcc and fcc phases of magnesium (it is the only one that remains hcp up to the melting point). Overall, it would appear that the column 2 elements display a wide variety of preferences for crystal structure, and hence bonding energy configurations. Is Mg unusual? Not really...

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.