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As I know, the melting points of Alkali metals decrease down to the group due to decreasing of the energy of the metallic bond.

enter image description here

But according to my understanding, after it has become to liquid the metallic bonds are not affecting furthermore because of the lattice has destroyed, so the boiling point affects only the atomic mass. Then according to my logic the boiling points should be increase down to the group. But the above graph shows the opposite of that.

What is the wrong in my logic. Please help me.

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    $\begingroup$ A molten metal is still a metal, with (more or less) the same metallic interaction between the constituent atoms.What gets destroyed upon melting is the crystalline lattice. $\endgroup$
    – Karl
    Nov 24, 2019 at 8:34

1 Answer 1

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You can explain the trend in terms of increased charge density, and ensuing charge repulsion between nuclei and between electrons as you go down the group.

An alternative explanation is provided in a libretext, which draws information from a popular educational chemistry site:

When any of the Group 1 metals is melted, the metallic bond is weakened enough for the atoms to move more freely, and is broken completely when the boiling point is reached. The decrease in melting and boiling points reflects the decrease in the strength of each metallic bond.

The atoms in a metal are held together by the attraction of the nuclei to electrons which are delocalized over the whole metal mass. As the atoms increase in size, the distance between the nuclei and these delocalized electrons increases; therefore, attractions fall. The atoms are more easily pulled apart to form a liquid, and then a gas. As previously discussed, each atom exhibits a net pull from the nuclei of +1. The increased charge on the nucleus down the group is offset by additional levels of screening electrons. As before, the trend is determined by the distance between the nucleus and the bonding electrons.

Trends in nuclear screening can be used to explain bonding but there are other ways to explain the trend, and such generalizations should in any case be understood to be simplified ways of describing what requires a more complex answer.

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  • $\begingroup$ Then why does the mp increase down the group in case of transition elements?@Buck_Thorn $\endgroup$
    – Shub
    Sep 28, 2022 at 5:44
  • $\begingroup$ @Shub I am not sure what would explain trends in transition elements. There might be a different explanation there. Also, my own interpretation of the trend (the first paragraph here) is not from a textbook, admittedly, as should be evident, but then in the later part of the answer I also present the textbook explanation. $\endgroup$
    – Buck Thorn
    Sep 29, 2022 at 11:49
  • $\begingroup$ See also: chemistry.stackexchange.com/questions/156254/… $\endgroup$
    – Buck Thorn
    Sep 29, 2022 at 11:52

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