Question on Melting Points of Group 17,18 and 1

So , I have done some searching and found that melting points of groups 17 and 18 *increase* going down the group whereas group 1 *decreases*.


However , the explanation seem to all be the same.


* Number of shells increase
* atomic radii increases
* Van der waals force between atoms increases
* more heat energy needed to overcome said forces


But for group 1, the explanation is that even when atomic radii increases , the metallic bond between atoms decreases and thus lower melting point. 


So , I'm a little confused about this metallic bond , and **why doesn't group 1 alkali metals also create Van Der Waals forces?**


I may be wrong on some parts so please do correct me.




**Metallic bonding** is a type of chemical bonding that rises from the electrostatic attractive force between conduction electrons (in the form of an electron cloud of delocalized electrons) and positively charged metal ions
**Van der Waals forces** include attraction and repulsions between atoms, molecules, and surfaces, as well as other intermolecular forces. 
The alkali metals have the largest size in their period. Consequently the distance between nuclei of one atom and the outermost electron of other atom is more and hence metallic bonding is less. But owing to their small size the distance between the nuclei of one halogen atom and the outermost electron of other atom is comparatively less. Hence they have show more metallic bonding.


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