Question

What is electronegativity?

Answer 1

Electronegativity is a way to quantify the observation that electrons are not equally shared between atoms participating in a chemical bond, e.g. in the H-F chemical bond between hydrogen and fluorine atoms, it is an experimental fact that the F end of the molecule is slightly negative, on average, and the H end slightly positive. We infer that the pair of electrons in the H-F bond is not shared equally, but spend more of their time around the F nucleus. We say that F is the more "electronegative" element, because F atoms will "hog" the electrons in the bond.

There are several ways to assign numerical values to the electronegativity of elements, but they're all fairly arbitrary and empirical. The most common, Pauling's, just assigns 4.0 to F, the most electronegative atom in the Periodic Table, then compares the polarity of bonds formed between atoms of each element. Francium has the lowest assign Pauling electronegativity, IIRC, at 0.7 or so.

The important facts are that electronegativity generally increases going up and to the right in the Periodic Table, from francium to fluorine, and excluding the noble gases, which generally do not participate in chemical bonding at all. It increases going up the table because atoms further down are larger, with many more core electrons layered underneath the valence (bonding) electrons. The increased distance between the nucleus and the valence electrons reduces the control the nucleus can exert over the bonding electrons. Electronegativity increases going across the Periodic Table because the effective nuclear charge increases. (Now you can ask what effective nuclear charge is.)