Trend: electronegativity

On the Pauling scale fluorine scores 3.98, the highest of any element. Caesium scores 0.79.

When two atoms share a pair of electrons, the sharing is rarely fair. One atom usually pulls the pair a little closer to itself, like the stronger person in a tug of war who drags the rope towards their side. Electronegativity is a number for how hard an atom pulls on shared electrons. It matters because it decides whether a bond will be shared evenly, shared unevenly, or not shared at all, which is the subject of the bonds course.

The Pauling scale

The American chemist Linus Pauling proposed the scale in 1932. He worked it out from how much energy it takes to break bonds between different atoms. The numbers have no unit and run from about 0.7 to just under 4. Fluorine is the highest at 3.98, then oxygen at 3.44, chlorine at 3.16 and nitrogen at 3.04. At the low end, caesium has 0.79 and francium is listed at 0.7. Carbon sits in the middle at 2.55 and hydrogen at 2.20.

The pattern

Electronegativity rises across a period and falls down a group, the reverse of the radius trend. This is no coincidence. A small atom with a high effective nuclear charge can bring a shared pair close to its nucleus and hold it hard. A large atom has its outer shell far from the nucleus and well shielded, so it pulls weakly. In the model the tiles climb from the left side to a peak at fluorine, then drop a little at the noble gases, which have no Pauling value because helium, neon and argon form no ordinary bonds.

A useful way to remember it: the most electronegative elements are the small nonmetals in the top right (F, O, N, Cl), and the least electronegative are the large metals in the bottom left. Metals with low electronegativity give electrons away; nonmetals with high electronegativity take them. When a metal from the far left meets a nonmetal from the far right, the difference is so large that the electron simply moves across.

A closely related trend is ionisation energy, the energy needed to pull one electron off a lone atom. PubChem lists it in electronvolts. It rises across a period and falls down a group, just like electronegativity: lithium 5.392, sodium 5.139, potassium 4.341 and caesium 3.894 going down group 1, and fluorine 17.423 and neon 21.565 at the right of period 2. Helium has the highest value of all, 24.587, because its two electrons sit in a tiny shell right next to the nucleus.

Using the difference

Subtract one electronegativity from the other and you get a rough guide to the bond. Two identical atoms, as in H₂ or Cl₂, have a difference of zero and share perfectly. Hydrogen and oxygen in water differ by 3.44 − 2.20 = 1.24, so the shared pairs sit closer to oxygen and the bond is polar. Sodium and chlorine differ by 3.16 − 0.93 = 2.23, large enough that chemists describe sodium chloride as ionic. There is no sharp cut-off. Textbooks often quote a difference of about 1.7 to 2 as the zone where bonds start to count as ionic, but it is a sliding scale.