Question

why magnesium has lower melting point than Calcium?​

Answer 1

Answer: let me explain

Explanation:

Excellent question. I’ll try to answer it. You see Calcium and Magnesium are group two elements. The Group 2 elements are all metals with metallic bonding, so you expect their melting points to be high. In metallic bonding, metal cations in a metal lattice are attracted to de-localized electrons.. Going down the group the following things happen -

the number of de-localized electrons remains the same ...

the charge on each metal cation stays the same at 2+, but ...

the ionic radius increases ...

so the attraction between the de-localized electrons and the metal cations decreases.

So, you would except them to show a smooth decrease in melting point. But, this is the graph -

Now, it is very clear that the melting point for magnesium is anomalously low. Now the answer lies in their crystal structures.

Here are their crystal structures -

Be - hcp at room temperature, bcc at melt.

Mg - hcp at room temperature, hcp at melt.

Ca - fcc at room temperature, bcc at melt.

Sr -, fcc at room temperature, bcc at melt.

Ba - bcc at room temperature and at melt.

(hcp - Hexagonal close packing, bcc - body centred cubic, fcc - face centred cubic)

Now, it is clear the reason lies in the crystal structure but I am not sure about it. I will write/edit it as soon as I figure it out.