Bromine has 4 shells, whereas iodine has 5. Positively charged protons are found in the nucleus of the atom. For bromine or iodine to react, each atom needs to gain an electron to fill up its shell so that it is in a more stable state. This makes it much easier for bromine to gain an electron to the outer shell.

Fluorine is the most reactive element in Group 7, and is even more reactive than chlorine. Fluorine’s outer shell is closer to the nucleus and has fewer filled shells between it and the nucleus, so the attraction for a new electron is greater and so it can gain an extra electron more easily.

Nonmetal reactivity decreases down a group because the nucleus’ ability to gain more valence electrons weakens due to more nuclear shielding. For Metals: the most reactive metals are those that can lose their valence electrons the most easily.

Rather, fluorine is the most reactive halogen. It reacts violently with almost all chemicals. It shows that the B.D.E value of iodine (151 kJ/mol) is less than that of fluorine (158 kJ/mol). So, iodine should be the most reactive halogen and not halogen.

Chlorine, a yellow-green gas, is more dense and less reactive than fluorine. Bromine is a dark red liquid. Iodine is a grayish black solid and is the least chemically active of the four; however, among the nonmetals only oxygen is more reactive than iodine.

Check out boron: it’s less electronegative than hydrogen [2.0 vs….Carbon is More Electronegative Than You Think.

So, Cl⋅ , chlorine radical, is less stable, and Cl− , chlorine ion, is more stable.