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.
The electronegativity of an element depends on three main factors:
- The number of protons the element has in its nucleus
- How far the electrons of the element which react with it lie from its nucleus
- How much screening is provided by the inner electrons of the element.
In the case of fluorine, there is no screening by inner electrons, it has many protons in its nucleus and the electrons of the electropositive elements that react with it are quite close to its nucleus.
These characteristics make it a highly electronegative element. Another thing we can see is that fluorine lies at the extreme upper right of the periodic table and electronegativity increases as we rise up any column and move from the left to the right in any row of the periodic table.
Fluorine has many protons in its nucleus , therefore it attracts its electrons a bit stronger, which makes the element reactive due to the fact that it's electrons are so close to it's nucleus.