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.