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Name:  Hira
Status: student
Grade: 9-12
Location:N/A
Country: N/A
Date: 12/19/2005


Question:
Why is that the melting point of magnesium is lower than that of calcium?


Replies:
I am sure you were expecting a monotonic variation in the melting points as you proceed from: Be, Mg, Ca, Sr, Ba, Ra in the group 2 elements. But the melting points are: 1551, 922, 1112, 1042, 1002, 973 kelvins, respectively. Melting is a very complicated phenomenon, despite some of the "trends" you find presented in texts. The crystal structure, that is the bonding in the solid state of an element (or compound), plays an important role in determining the melting point. The elements: Be, Ca, and Sr have several crystal structures. The element Mg has a hexagonal close packed structure in the solid state, while Ba and Ra have body centered cubic crystal structures. The stability of the solid state has a profound effect on the melting temperature of a substance. Some substances, for example phosgene (O=C(Cl)2), has three different melting points depending upon which crystal phase is melting! So it is not too surprising that melting points do not follow simple trends. One need not go any further than plain old water to make one very cautious about simple trends in the temperature of changes of physical state.

Vince Calder



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