Ask A Scientist Chemistry Archive

name         Hira
status       student
grade        9-12
location     N/A

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