why has fluorine the highest electronegativity in the modern periodic table?

Electronegativity is a measure of how strongly the nucleus
of a given element attracts electrons toward itself.  On the periodic table there are
two trends:  as you go from left to right in a given period the electronegativity
increases; and as you go down a group the electronegativity
decreases.

Both trends are due to the structure of the
atom.  Remember that the nucleus is small, dense, and positively charged while the
surrounding electrons are negatively charged and relatively diffuse around the nucleus.
Thus, as you go across a period you are adding more and more protons to the nucleus,
increasing the positive charge in a relatively small volume.  You are adding an equal
number of electrons, but because of the increasing positive charge of the nucleus
electrons are attracted more and more strongly toward the nucleus. This also explains
why the atomic radius decreases as you go across the
period.

As you go down a group you are adding an additional
layer of electrons but they are farther from the nucleus. Consider Coulomb's law:  F =
kQ1Q2/d^2.  This gives the force between two charges (Q1,Q2) and the distance between
the charges (d).  Because the distance term is squared the force drops quickly as the
electrons are farther from the nucleus.  That is again why Flourine is the most
electronegative element. You have electrons in the S1, S2, and P1 levels which are the
closest to the nucleus so the distance is small and the force
greater.