What is electronegativity?

A tug-of-war over electrons

When two atoms share a pair of electrons, the sharing is rarely fair. Electronegativity measures how hard an atom pulls on shared (bonding) electrons — the famous Linus Pauling scale puts fluorine at the top (3.98 on modern tables) and francium and caesium at the bottom (around 0.7). Oxygen is greedy (3.44), nitrogen moderately so (3.04), carbon keeps it fair (2.55), and metals barely pull at all.

It is a property without units because Pauling built it as a comparison: take a bond, measure its actual strength, compare it with the average of the two hypothetical homonuclear bonds, and the extra energy is credited to the tug-of-war. The scale is relative, but its consequences are extremely practical.

The trend, and the two reasons behind it

Electronegativity rises to the right across a period and up a group — fluorine is the champion of the whole table. Two causes, both worth writing in exams: across a period, protons pile into the nucleus while the electrons stay in the same shell, so the pull on bonding electrons tightens; up a group, the outer electrons sit closer to the nucleus, with fewer filled shells shielding them, so the pull tightens again. Reverse both directions and you get the metal corner: bottom-left, minimal pull, electrons surrendered freely.

What the difference between two values predicts

Subtract the two electronegativities of a bonded pair and the gap tells you the bond type: near zero (say ΔEN < 0.4) → nonpolar covalent, electrons shared honestly ($\mathrm{Cl_{2}}$, $\mathrm{O_{2}}$); moderate (0.4–1.7) → polar covalent, shared unfairly ($\mathrm{H_{2}O}$, $\mathrm{HCl}$); large (> 1.7) → ionic, the electron transfers outright ($\mathrm{NaCl}$: 3.16 − 0.93 ≈ 2.2). The boundaries are guides, not walls — but the logic is exam gold, because it converts one subtraction into a full bonding discussion.

Water's whole biography depends on this: oxygen (3.44) versus hydrogen (2.20) gives a polar O–H bond; two polar bonds on a bent shape give a polar molecule; polar molecules stick together, which is why water is liquid at room temperature, dissolves salt, climbs plant stems and makes life possible. One trend, read properly, explains a planet.

Where the values misbehave

Noble gases mostly have no listed values at all — with a full shell they rarely bond, so Pauling's comparison has nothing to measure (xenon and krypton have approximate values from their rare compounds). Gold is a quiet surprise: at 2.54 it out-tugs everything nearby, one reason gold resists corrosion and stays museum-shiny for millennia.

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