Group 7, Halogens.

The atomic radius of group 7 atoms increases as you go down the group. as each element as you go down the group has one more shell of electrons compared to the one above.

Electronegativity is the measure of an atom's ability to pull elecrons towrds itself, within a covalent bond. Factors that affect it are the atraction between the nucleus and the electrons in the outer shell, also, the distance between the nuclues and the outer electrons, and the shielding between them. This means, that as you go /down/ the group, the atoms will get less electronegative, as the atoms are bigger as you go down the group, meaning there are more shells, and there is therefore, a further distance, and more shielding taking place.

The oxidising ability of the group 7 elements increases as you go up the group. So, fluorine is the most powerful oxiding agent. You can tell this from displacement reactions, as none of the other hallides can displace from fluorine, and only fluroine can displace from chlorine, and so on.