[ramboacid]

Recently I came across this question:

Which element has the highest melting point:
A) Na
B) K
C) Mg
D) Ca

I guessed C) Mg, but the answer is D) Ca. Is there a logical explanation for why this is the case, that I could use to reason this out on a test if it came up? Or is it just something you have to remember?

I've already read through some previous posts on the forum and gleaned that the trend in melting point for transition metals is determined by the highest oxidation state, because that determines how many electrons each metal atom can donate to the electron sea in metallic bonding. I would guess this also applies to the Group I and II metals; since Group II metals are found in higher oxidation states than Group I metals, they should have stronger metallic bonding and thus have higher melting points. Therefore I can rule out Na and K.

At this point, I would be stuck between choosing Mg or Ca. I tried reasoning it out using specific heats and atomic radii. Since Ca is more massive than Mg, it should have a lower specific heat than Mg. I tried extrapolating this to say that Mg could thus absorb more heat energy per degree Celsius than Ca, and so Ca would have a lower melting point because its temperature would increase more quickly. Assuming temperature is proportional to kinetic energy, Ca atoms would break their intermolecular bonds more easily than Mg atoms and thus have a lower melting point. Also, since Ca has a larger atomic radius than Mg, I would assume that the intermolecular forces between Ca atoms are already weaker than those between Mg atoms, which further points towards Ca having a lower melting point than Mg.

However, this is not the case as Ca does in fact have a higher melting point than Mg. Am I missing anything in my reasoning? Is my reasoning just completely incorrect in itself? Or is this just a special case, an anomaly in periodic trends?

Any feedback would be appreciated.

[nannon27131]

Its easy to know that:
    *  Metals generally possess a high melting point.
    * Most non-metals possess low melting points.
    * The non-metal carbon possesses the highest boiling point of all the elements. The semi-metal boron also possesses a high melting point.

But when it comes to distinguishing between metals it becomes difficult. You have to look at bond strength.

Sodium has the electronic structure 1s22s22p63s1. Meanwhile Magnesium has the outer electronic structure 3s2. Both of these electrons become delocalised, so the "sea" has twice the electron density as it does in sodium. The remaining "ions" also have twice the charge (if you are going to use this particular view of the metal bond) and so there will be more attraction between "ions" and "sea". 
So with magnesium, you end up with stronger bonds and so a higher melting point than say sodium.

Source:http://www.chemguide.co.uk/atoms/bonding/metallic.html