For Group 1 and Group 17 elements, how does their position on the Periodic Table reflect their chemistry?
Group I elements are metals; Group 17 elements are non-metals.
As alkali metals, the Group I elements are non-molecular solids (almost all of them are!). On the other hand, the Group 17 elements are molecular, with discrete molecular boundaries as gases, liquids, or solids. Metals are reducing, and non-metals are oxidizing, and hence the alkali metals form salts with the halogens.