Why does electronegativity decrease as you go down a group?
Electronegativity is a measure of the ability of an element to attract bonding electrons to itself. The attraction is due to the natural attraction between the positive nucleus and the negative electrons around each atom.
As the size of the nucleus of an element increases, it also adds additional electrons around itself. These electrons go into orbitals that are further and further from the nucleus. These outer electrons act as a shield against other electrons, repelling them away. In addition, if you look at Coulomb's Law, it tells you that the force between two charges decreases as square of the distance between them increases. Thus, because potentially bonding electrons are kept further from the nucleus, the attraction of the nucleus on them gets weaker and weaker.
That is why Flourine is the most electronegative atom. It has a small,dense core of neutrons with only a relatively thin layer of electrons surrounding the nucleus.
Electronegativity decreases as you go down a group within the Periodic Table. This is because of the longer distance between the nucleus and the Outer Valence electron shell within an atom. This therefore decreases the attraction.