Electronegativity

Definition. Electronegativity is the relative tendency of an atom, within a covalently bonded molecule, to attract the shared pair of electrons toward itself.

Why it needs a scale. Unlike atomic radius or ionisation energy, electronegativity cannot be measured directly -- it is not a property of an isolated atom, only of a bonded one. Several scales exist to assign it a number; the most widely used is Linus Pauling's. Pauling anchored the scale by arbitrarily assigning hydrogen an electronegativity of 2.1 and fluorine 4.0, then calculated the electronegativity difference between any other bonded pair A-B from their bond dissociation energies:

where $E_{AB}$, $E_{AA}$ and $E_{BB}$ are the bond dissociation energies (in kcal) of the AB, A2 and B2 molecules respectively. Because this method starts from bond energies, the electronegativity value obtained for an element is not truly a fixed constant -- it depends on exactly which element it is bonded to -- but the Pauling values in everyday use (Table 3.15) are averaged, practical numbers that predict the polarity/nature of a bond well.

Trend across a period: electronegativity generally increases, left to right (Figure 3.6). This follows directly from the atomic-radius trend (Section 3.5.1): as the radius shrinks across a period, the attraction between the nucleus and the valence electrons grows, and with it the atom's tendency to pull a shared electron pair toward itself. …