Electronegativity is the tendency of an atom of an element to attract electrons towards itself.
Florine in the 2nd period is the most electronegative element. It has the strongest tendency to attract an electron and become an ion to get the stable configuration of electrons.
In a group as one moves from top to bottom, the size of the atom increases, and so the ability to attract an electron decreases. So the distance between the nucleus and the outer most electron decreases.
At the top of the group, the electronegativity is highest.