Electronegativity is a measure of how easily an atom attracts a pair of electrons to form a chemical bond. A high electronegativity value means an atom readily attracts electrons to form a chemical bond with another atom. A low electronegativity value means an atom readily donates electrons to form a bond or is electropositive.
While there are charts of electronegativity values for elements of the periodic table, there is no true single electronegativity value for an atom. Rather, it depends on the other atoms in a molecule and also depends on the nuclear charge and number of electrons. The most common method of calculating electronegativity is the Pauling scale, which was proposed by Linus Pauling. The Pauling scale runs from 0.79 to 3.98. The Pauling scale is dimensionless, but sometimes the values are cited in Pauling units.
Most Electronegative and Most Electropositive Elements
The most electronegative element is fluorine, with an electronegativity value of 3.98 on the Pauling scale. The least electronegative or most electropositive element is cesium, which has a value of 0.79. However, francium is probably even more electropositive than cesium because it has a higher ionization energy. Francium’s electronegativity value is estimated to be around 0.79, but it has not been measured empirically.
Electronegativity and Chemical Bonding
Comparing electronegativity values allows prediction of the type of chemical bond two atoms will form. Atoms with the same electronegativity values (e.g., H2, N2) form covalent bonds. Atoms with slightly different electronegativity values (e.g., CO, H2O) form polar covalent bonds. All hydrogen halides (e.g., HCl, HF) form polar covalent bonds. Atoms with very different electronegativity values (e.g., NaCl) form ionic bonds. Note that electronegativity does not help in predicting whether or not a chemical bond will actually form. Argon has a high electronegativity value, yet it’s a noble gas that forms few chemical bonds.
Electronegativity Periodic Table Trend
- Electronegativity increases moving left to right across a period, from the alkali metals to the halogens. The noble gases are an exception to the trend.
- Electronegativity decreases moving down a periodic table group. This is because the distance between the nucleus and the valence electrons increase.
- Electronegativity follows the same general trend as ionization energy. Elements with low electronegativities tend to have low ionization energies. Similarly, an atom with a high electronegativity tends to have a high ionization energy.
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- Mullay, J. (1987). Estimation of atomic and group electronegativities. Structure and Bonding. 66. pp. 1–25. doi:10.1007/BFb0029834. ISBN 978-3-540-17740-1.
- Pauling, Linus (September 1, 1932). “The Nature of the Chemical Bond. IV. The Energy of Single Bonds and the Relative Electronegativity of Atoms.” J. Am. Chem. Soc. 54, 9, 3570-3582. ACS Publications.
- Pauling, Linus (January 31, 1960). The Nature of the Chemical Bond and the Structure of Molecules and Crystals: An Introduction to Mode (3rd ed.). Cornell University Press.