What is electronegativity?

Electronegativity is the power of an atom to attract the electron density in a covalent bond towards itself. 
It depends on1.    The nuclear charge of the element2.    The distance between the nucleus and outer shell of electrons3.    The shielding of the nuclear charge by electrons in the inner shells. 
For example, Fluorine is the most electronegative element for the following reasons:·       Fluorine’s electron configuration is 1S²2S²2P⁵. The optimal configuration of the 2P orbital has 6 electrons, so the Fluorine strongly attracts the additional shared electron in a covalent bond towards itself. Elements in lower groups to Fluorine do not have as strong an attraction, as they have fewer electrons in their 2P orbital and so do not as easily attract the electron in the covalent bond as it will not complete the orbital.
·       There is only a short distance between Fluorine’s nucleus and the 2P orbital, meaning that the 9 protons (with positive charge) can strongly attract and “hold on” to the electrons in its shells, and also attract a further electron. As you go down the periodic table, the electronegativity will decrease as the distance between the nucleus and the outer shell of electrons and the covalent bond increases, so there is not as great an attraction.
·       Fluorine only has 2 electrons in the 1S orbital to contribute to the ‘shielding’ of the nucleus’ charge (this is where it forms a sort of barrier of negative charge to reduce the attraction of other electrons to the protons in the nucleus). As you go down the periodic table, there is more shielding as electrons are filling higher shells, therefore reducing the elements’ electronegativity.