Chemistry – The Periodic Table - Group VII properties | e-Consult

The halogens' reactivity decreases down the group (F > Cl > Br > I). Therefore, the relative reactivity of the halogens with sodium will be: Na > Cl > Br > I.

This is because reactivity increases up a group in the halogen reactivity series. Sodium is highly reactive, and it will readily lose an electron to form a positive ion (Na⁺). The halogens will readily gain an electron to form a negative ion (X^-). The greater the tendency of the halogen to gain an electron (and therefore the higher its electronegativity), the greater its reactivity with sodium. Fluorine is the most reactive halogen, followed by chlorine, bromine, and then iodine. The weaker the bond between the halogen atoms, the easier it is for the halogen to lose an electron and react with sodium.