Why Does Electronegativity Vary Down Groups 13–17?

Electronegativity generally decreases down a group because the atoms become larger, so a bonding pair of electrons is farther from the nucleus. But the trend is not perfectly regular in groups 13–16. The halogens in group 17 are commonly presented as a clearer case: their tabulated Pauling electronegativities decrease from fluorine down the group.

The difference is a matter of how strongly other effects disrupt the broad size trend—not a special rule that makes group 17 immune to exceptions or uncertainty.

The usual down-group trend

Electronegativity describes how strongly an atom attracts shared electrons in a chemical bond. It is not measured on a single absolute scale; the values most often quoted in introductory chemistry are on the Pauling scale.

Going down a group adds occupied electron shells. The valence electrons are typically farther from the nucleus, and inner electrons partly shield them from its attraction. These effects tend to make an atom less electronegative.

However, the increase in atomic size is not the only factor. Inner d and f electrons do not shield outer electrons as effectively as the simplest model suggests. In heavier p-block elements, this imperfect shielding can increase the effective nuclear attraction and partly offset the effect of added shells. The resulting electronegativity values may flatten or show a reversal.

What the values show

These representative Pauling values illustrate the contrast. Small differences can vary by reference table and rounding; astatine’s value is especially uncertain.

Group	Elements from top to bottom	Representative Pauling values
13	B, Al, Ga, In, Tl	2.04, 1.61, 1.81, 1.78, 1.62
14	C, Si, Ge, Sn, Pb	2.55, 1.90, 2.01, 1.96, 2.33
15	N, P, As, Sb, Bi	3.04, 2.19, 2.18, 2.05, 2.02
16	O, S, Se, Te, Po	3.44, 2.58, 2.55, 2.10, 2.00
17	F, Cl, Br, I, At	3.98, 3.16, 2.96, 2.66, about 2.2

Groups 13–16 are not all irregular in the same way. For example, gallium’s value is higher than aluminium’s, and lead’s commonly quoted value is higher than tin’s. By contrast, the changes from phosphorus to arsenic, and from sulfur to selenium, are small decreases. The pattern depends on the element pair; it is not accurate to say that electronegativity always increases down groups 13–16.

For group 17, the commonly listed values descend at each step. The fall from fluorine to chlorine is especially large, and the differences lower down are smaller. Astatine is rare and radioactive, so its electronegativity is not as securely established as the values for the lighter halogens.

A worked comparison

Compare aluminium with gallium, then chlorine with bromine:

• In group 13, Al is about 1.61 and Ga about 1.81. Thus electronegativity increases slightly from Al to Ga, contrary to the simplest down-group expectation.
• In group 17, Cl is about 3.16 and Br about 2.96. Thus electronegativity decreases from Cl to Br, as expected from the usual trend.

The values support the observed order, but the explanation is not simply that one group follows a different law. Atomic size, shielding, and the electronic structure of each element all contribute. Across a short step such as Al to Ga, the shielding and contraction effects can be large enough to outweigh the size-related decrease.

Why group 17 looks more regular

Fluorine’s very high electronegativity makes the start of the halogen series distinctive, and the established values for chlorine, bromine, and iodine continue downward. Although inner d and f electrons also affect heavier halogens, those effects do not reverse the commonly tabulated order for this group. The trend is therefore regular in the usual tables, not because d- and f-electron effects disappear, but because they do not overcome the overall decrease in this sequence.

Remember that electronegativity is a comparative, scale-dependent quantity. Different scales and reference tables can give slightly different numbers, and values for heavy elements may be estimates. Use a consistent scale when comparing elements.

Common mistakes

• Treating the trend as an absolute rule. Electronegativity often decreases down a group, but individual steps can be nearly flat or reverse.
• Assuming all groups 13–16 behave alike. Each group has its own pattern; some changes are small, while others are more pronounced.
• Blaming only atomic size. Poor shielding and changes in effective nuclear attraction also matter, especially for heavier p-block elements.
• Reporting astatine’s value as exact. Its tabulated electronegativity is approximate and less certain than those of the lighter halogens.
• Mixing scales. Numerical values from different electronegativity scales are not directly interchangeable.