Electronegativity is the tendency of a (neutral) atom to attract negative charge (electrons).
    The electronegativity of the atoms depends on:
  1. The distance between the nucleus and the outer shell
  2. The (postive) charge of the nucleus (= number of protons)
Here we must apply the Law of Coulomb. In some schools this law is not treated anymore. It used to be part of Physics in secondary education. With the Law of Coulomb you can better understand how forces between atoms and ions take place.
You must just know the Law of Coulomb; no doubt.
You must imagine that two charges attract or repell each other twice as hard if a charge becomes twice as big. But if the distance between those charges becomes twice as large, than the attraction of repelling forces become four times smaller.

F is the attraction (or repelling) force between charges.

Imagine an atom: there is a positive charge and - at some distance - negative electrons.
The nucleus attracts the negative charges (consider in particular those charges at the outer side, so at maximum distance from the nucleus).
The attraction power depends on Q1, Q2 and r (the charges and the distance)
Q symbolises the charges (of the nucleus and the electrons); the value of r in this kind of calculations is about similar withthe atom rays.
In atoms with a relative strong postive nuclear charge (G is big) and a relative small atom ray (r is small) the value of F will be big.
In the case of atoms you must indicate attraction force F with the E of electronegativity.

how big are atoms, relatively:

atoms of H are very small (r = <<) Ba is big (r = >>)