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It is written in the third point ( shown in the picture) that presence of d orbitals results in stronger metallic bond.

1) We check the strength of metallic bonding to compare the melting and boiling point in case of metals. According to the general trend, melting and boiling point decreases down in the group( in s block) because the strength of metallic bonding decreases.

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2) Presence of d orbitals decrease the size of atom thus increasing the electrostatic attraction between the delocalized electrons in the metal lattice and the cation and as a result, increasing metallic bonding. But magnesium IS IN A DIFFERENT PERIOD.

Can the ineffective shielding by d electrons be so much effective in decreasing the size of cation?

3) We didn't use any of this d orbital argument in comparing melting and boiling point of alkali metals. It decreased down the group following the expected trend.

How should we explain this fact ?

Arishta
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  • Explanation based on d-orbitals is rather questionable. Do a search on SE for "magnesium melting point", this question has been asked numerous times. – orthocresol Jan 08 '17 at 15:40
  • @Nilay You have >3,000 rep, so you can already vote to close as a dupe by yourself. (Click on "close" next to "edit".) – orthocresol Jan 08 '17 at 15:43
  • I read some of the answers posted earlier on this topic, including the one that Nilay Ghosh mentioned but I don't really find them convincing. What should I do if I don't find the earlier answers convincing? – Arishta Jan 08 '17 at 15:45
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    http://chemistry.stackexchange.com/questions/27143/why-does-magnesium-have-the-lowest-melting-point-of-all-earth-alkalis – Nilay Ghosh Jan 08 '17 at 17:15
  • @Blue Put a bounty on the existing question. – orthocresol Jan 08 '17 at 17:42

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