Why does electronegativity increase as effective nuclear charge increases?

I know that electronegativity is the ability to attract shared electrons and that effective nuclear charge is the pull of the nucleus on outer electrons based on my notes.

The effective nuclear charge can be thought of the charge of the nucleus minus the charge of the core electrons. For an element such as fluorine, the nuclear charge is $+9$ and the core electrons have a charge of $-2$ so the effective nuclear charge is $+7$. Similarly for carbon it would be $+6 - 2 = + 4$.

Now let’s assume a $\ce{C-F}$ bond. There are two electrons in this bond, each charged $-1$. On the left, from the carbon atom, they ‘see’ a charge of $+4$; on their right, from the fluorine atom, they ‘see’ s charge of $+7$.

Electrostatic interactions are greater at shorter distance and at greater charge separation, so the interaction (i.e. force) the bonding electrons experience from fluorine’s side is almost double that from carbon’s side. (The carbon and fluorine atoms can be considered to be roughly of the same size.) It should now be clear why the electrons are drawn to the fluorine.

Electronegativity is just a name humans gave the observed effect of ‘this atom can pull electrons towards it better than others’. It takes a look at the result of the above process and rationalises it. The effective nuclear charge approach is the underlying physical force.

Note: This explanation is extremely simplified, does not take any quantum effects or the like into consideration and should thus only be used at introductory levels.

I think Lewars textbook gives a good outline as to what electronegativity is. The thing with electronegativity is that that it is vague. Anyhow, here is what Lewars says:

From this viewpoint electronegativity of a species is the drop in energy when an infinitessimal amount (infinitessimal so that it reamins the same species) of electronic charge enters it. It is a measure of how hospitable an atom or ion, or a group or an atom in a molecule, is to the ingress of electronic charge, which fits in with our intuitive concept of electronegativity.

You could put it this way: the nucleus attracts the electron more if it has a high effective nuclear charge. Remember that nuclear charge is positive, and that electrons are negative https://en.wikipedia.org/wiki/Electric_potential_energy. Thus an electronegative atom/ion/etc "gains more energy" when it pulls an electron towards it. And the reason it "gains more energy" is because it has high effective nuclear charge.

• What's "Lewars textbook"? – Wildcat Oct 3 '15 at 8:53
• "An Introduction to Computational Chemistry" Errol G. Lewars – CoffeeIsLife Oct 4 '15 at 1:17

Electronegativity can be thought of how much an atom "wants" electrons.

An atom that wants electrons is one that has a high positive charge, or effective nuclear charge. Effective nuclear charge is the pull the nucleus has on outer electrons (taking into account the repulsion of electrons in the atom.)

The higher the effective nuclear charge, the more the nucleus wants electrons, which is why atoms high in one is high in the other and vice versa.