Why the m.p/ b.p. of Ca > Mg?

The melting points (m.p.) and (b.p.) of the Group II elements are shownbelow :

Be: m.p. 1278°C, b.p. 2970°C
Mg: m.p. 650°C, b.p. 1110°C
Ca: m.p. 850°C, b.p. 1487°C
Sr: m.p. 769°C, b.p. 1384°C
Ba: m.p. 729°C, b.p. 1637°C

Going down Group II, the melting points and boiling points are generallydecreasing. This can be explained by the fact that the strengths of metallicbond are decreasing down Group II. Thisis because the increase in ionic radius when going down the group leading tothe decrease in attraction between the delocalized electrons and the cations.
(Never use number of protons to explain the strengths of metallic bond of thesame group metals, because the increase in the number of protons is cancelledby the increase in the screening effect due to the increase in number of innerelectronic shells.)

Although in general the melting point and boiling point decrease going down thegroup, the melting point and boiling point for Mg is anomalously low. This isbecause Be and Mg have different metallic structures from the other elements inthe group:
• Be and Mg have a hexagonal close-packed structure.
• Ca and Sr have a face-centred cubic structure.
• Ba has a body-centred cubic structure. (The boiling point of Ba is alsoanomalously high.)

Many textbooks provide the above explanation. However, the above explanation isNOT a complete explanation.

2013-07-29 03:08:28 補充：
You are right. That is why I said the explanation is NOT 'complete'. However, this is the only explanation in many AL textbooks.

you have missed a crucial point that the protons number of Ca is 8 more than that of Mg, hence there are more positively charged protons to attract the delocalized (valence) electrons, the strength of metallic bond is stronger among Ca atoms than that of the Mg, hence its b.p and m.p is higher than Mg