Re: Periodic trend in electronegativity

Electronegativity is partly based on ionization energy, so you might expect that they go along with each other. I often explain electronegativity as "a tendency to hold electrons more tightly."

So the question may be reduced to "Why do atoms hold their electrons more tightly as one moves from left to right accross the Periodic Table?"

You probably know that electrons occupy "shells" at sequential levels in an atom. To a first approximation, all electrons in the same shell are the same distance from the nucleus and are not "shielded" from that nucleus by any of the other electrons in that shell. However, an electron in a higher shell is shielded from the nucleus by electrons in lower shells. This means that each lower-shell electron cancels one unit of positive charge for electrons in higher shells.

The next thing to remember is that, as you move from lower to higher atomic number (that is, from left to right in the Periodic Table), the amount of positive charge in the nucleus increases.

Finally, remember that the strength of an electrostatic attraction (between positive and negative charges) depends only on the distance and the charge of the two objects being considered.

To a first approximation, we can neglect electron-electron repulsion as a contributor to ionization energy, because the electrons are zipping around each other so fast that, on average, electrons are just as likely to be pushed toward the nucleus as away from it by any other electron in the same shell.

Therefore, the only thing we need to consider for ionization energy is the amount of positive charge any particular outer electron "sees" in the nucleus. Again, this charge is affected only by inner electrons, not by the presence of other out electrons!

For the first example, consider lithium. Lithium has three protons in its nucleus; it also has three electrons, two in the first (innermost) shell and one in the second shell, which is called the valence shell since it is the outermost shell in the atom. The single valence electron sees only one unit of positive charge (the inner-shell electrons see three units), and so it is rather loosely held. Therefore the ionization energy of lithium is rather low, only 5 eV.

Now consider beryllium. Beryllium has four protons and four electrons. However, since the inner shell can hold only two electrons, beryllium has two electrons in its valence shell, and each valence electron sees two units of positive charge. You would expect beryllium's valence electrons to be held more tightly, and they are: beryllium's ionization potential is 9 eV.

We see this trend across the entire row: each atom adds one more proton and one more electron, but since only two electrons will fit into the inner shell, each valence electron sees one more unit of positive charge than in the previous element. This is reflected in both ionization potentials and electronegativities of the elements of the 2d Period:

        Dan Berger
        Bluffton College
        http://cs.bluffton.edu/~berger/