How does electron configuration effect electronegativity?

If you know enough to ask this question you know the basics of electron configuration. You know, for example that the n=1 electron shell has only two electrons in it and that it is very close to the nucleus.

Let's look at the n=2 electron shell. That shell covers the elements from lithium to neon. When you open a new electron shell, you actually only do it once and it opens a whole bunch of possible places for electrons to hang out. so with lithium, you have 2 electrons in the n=1 shell (#1s^2#) and one electron in the n=2 shell (#2s^1#).

With me so far? Good! Now consider what is holding that electron in place. It is the attraction to the positive charge of the three protons in the lithium nucleus. However, the single 2s electron is also repelled by the two electrons in the n=1 shell. They, in effect, block the 2s electron from feeling the attraction of all three protons. Actually, because the two n=1 electrons are closer to the nucleus than the 2s electron, it only feels as if it were attracted to one proton!

This concept is called the effective atomic charge. So for lithium, the effective atomic charge experienced by the 2s electron is +1. Now, when you go on to beryllium, you are adding another electron to the n=2 shell, so it is neither further from the nucleus nor closer to the nucleus than the first n=2 electron. Therefore it doesn't block the attraction of the nucleus. Each of the n=2 electrons feels an attraction of +2, so both are held more strongly in beryllium than the one electron in lithium. The effective atomic charge of beryllium for the n=2 electrons is +2.

Now let's look at fluorine! By the same reasoning, fluorine has an effective atomic charge for n=2 electrons of +7! Electrons really like (feel a very strong attraction for) being in the n=2 shell of fluorine!

But wait! We know that there are actually eight slots for electrons in the n=2 shell. So that means there is room for another electron! And that extra electron will feel the same attraction for the fluorine nucleus as the first seven n=2 electrons! It is neither farther away nor closer to the nucleus than the other seven.

This is the origin of electronegativity: electrons will tend to migrate towards atoms with high effective atomic charges, providing there is an open spot for them in the outermost shell of the atom.

Each electron shell is further away from the nucleus than the one before it. And the attraction between the effective atomic charge and the electrons weakens with distance, so, for example, oxygen, with an effective atomic charge of 6 has almost the same attraction for electrons as chlorine, with an effective atomic charge of 7 because the chlorine n=3 shell is further away than the oxygen n=2 shell.