Electronegativity Definition and Examples

Chemistry Glossary Definition of Electronegativity

This periodic table indicates each element's electronegativity.
This periodic table indicates each element's electronegativity. Todd Helmenstine

Electronegativity is the property of an atom which increases with its tendency to attract the electrons of a bond. If two bonded atoms have the same electronegativity values as each other, they share electrons equally in a covalent bond. However, usually the electrons in a chemical bond are more attracted to one atom (the more electronegative one) than to the other. This results in a polar covalent bond. If the electronegativity values are very different, the electrons aren't shared at all. One atom essentially takes the bond electrons from the other atom, forming an ionic bond.

Avogadro and other chemists studied electronegativity before it was formally named by Jöns Jacob Berzelius in 1811. In 1932, Linus Pauling proposed an electronegativity scale based on bond energies. Electronegativity values on the Pauling scale are dimensionless numbers that run from about 0.7 to 3.98. The Pauling scale values are relative to the electronegativity of hydrogen (2.20). While the Pauling scale is most often used, other scales include the Mulliken scale, Allred-Rochow scale, Allen scale, and Sanderson scale.

Electronegativity is a property of an atom within a molecule rather than an inherent property of an atom by itself. Thus, electronegativity actually varies depending on an atom's environment. However, most of the time an atom displays similar behavior in different situations. Factors that affect electronegativity include the nuclear charge and the number and location of electrons in an atom.

Electronegativity Example 

The chlorine atom has a higher electronegativity than the hydrogen atom, so the bonding electrons will be closer to the Cl than to the H in the HCl molecule.

In the O2 molecule, both atoms have the same electronegativity. The electrons in the covalent bond are shared equally between the two oxygen atoms.

Most and Least Electronegative Elements

The most electronegative element on the periodic table is fluorine (3.98). The least electronegative element is cesium (0.79). The opposite of electronegativity is electropositivity, so you could simply say cesium is the most electropositive element. Note older texts list both francium and cesium as least electronegative (0.7), but the value for cesium was experimentally revised to the 0.79 value. There is no experimental data for francium, but its ionization energy is higher than that of cesium, so it is expected that francium is slightly more electronegative.

Electronegativity as a Periodic Table Trend

Like electron affinity, atomic/ionic radius, and ionization energy, electronegativity shows a definite trend on the periodic table.

  • Electronegativity generally increases moving from left to right across a period. The noble gases tend to be exceptions to this trend.
  • Electronegativity generally decreases moving down a periodic table group. This correlates with the increased distance between the nucleus and the valence electron.

Electronegativity and ionization energy follow the same periodic table trend. Elements that have low ionization energies tend to have low electronegativities. The nuclei of these atoms don't exert a strong pull on electrons. Similarly, elements that have high ionization energies tend to have high electronegativity values. The atomic nucleus exerts a strong pull on electrons.