The melting and boiling points for the Group II elements are as follows: $$\begin{array}{ccc} \text{Element} & \text{Melting point / }\mathrm{^\circ C} \\ \hline \ce{Be} & 1289 \\ \ce{Mg} & 650 \\ \ce{Ca} & 842 \\ \ce{Sr} & 769 \\ \ce{Ba} & 729 \end{array}$$ (source: Greenwood & Earnshaw, Chemistry of the Elements 2nd ed., p 112)

As you can see, the overall trend is decreasing, but magnesium is an exception. Just like the alkali metals, melting point should decrease for the heavier elements of the group because of the weaker bonding. Why is magnesium different?

The other trends for this group, such as density, atomic radius, and first ionization energy follow the expected pattern with no exceptions.

  • $\begingroup$ I am guessing it might be to do with the structure of the metals. Be and Mg are hcp, Ca and Sr ccp, Ba and Ra bcc. $\endgroup$ Commented Apr 15, 2016 at 14:17
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    $\begingroup$ As a further comment, counting on trends in the periodic table will lead you in many wrong directions. Expect the unexpected. $\endgroup$
    – Jon Custer
    Commented Apr 15, 2016 at 14:48
  • $\begingroup$ Mg has it's special properties, but the m.p. of Be is the one that is actually out of line, as are many properties of the elements in the second period. Plus Be lies nicely on the diagonal in the periodic system, where some properties are metallic, and some not. $\endgroup$
    – Karl
    Commented Apr 15, 2016 at 16:42