Group 1 elements have an $ns^1$ valence shell electronic configuration. They are reactive metals with low ionization enthalpies and lose their outermost electron readily to form a 1+ ion. As we go down the group, the metallic character and reactivity increase, resulting in the order Li < Na < K < Rb < Cs.

For Group 17 elements, the electronegativity values decrease down the group (F = 4.0, Cl = 3.0, Br = 2.8, I = 2.5). A higher electronegativity indicates a greater tendency to gain electrons, making the element more reactive. Therefore, the reactivity order is F > Cl > Br > I.