why alkali metals have low melting and boiling points
It is because metal atoms are held together by the electromagnetic attraction from the positive ions to the delocalised electrons.
Alkali metal
This is because metal atoms are held together by the electromagnetic attraction from the positive ions to the delocalised electrons.
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Alkali metal [SEP] Melting and boiling points
This is because metal atoms are held together by the electromagnetic attraction from the positive ions to the delocalised electrons. As the atoms increase in size going down the group (because their atomic radius increases), the nuclei of the ions move further away from the delocalised electrons and hence the metallic bond becomes weaker so that the metal can more easily melt and boil, thus lowering the melting and boiling points. (The increased nuclear charge is not a relevant factor due to the shielding effect.)