D2.6 Electronegativity
Electronegativity (EN) is the tendency of an atom in a molecule to attract bonding electron density. Electronegativity values for most elements are shown in the periodic table in the Activity below; they also are tabulated in the appendix.
Electronegativity, Electron Affinity, and Ionization Energy
These three properties are all associated with an atom gaining/losing electrons. Electron affinity and ionization energy are experimentally measurable physical quantities.
Electron affinity (EA) is the energy change when an isolated gas-phase atom acquires an electron; it is usually expressed in kJ/mol.
Ionization energy (IE) is the energy that must be transferred to an isolated gas-phase atom to remove an electron; it is also typically expressed in kJ/mol.
Electronegativity describes how strongly an atom attracts electron density in a bond. It is calculated, not measured, has an arbitrary relative scale, and has no units.
Based on the definitions of electronegativity, electron affinity, and ionization energy, think about the similarities and differences among these three properties. In your course notebook, write two or three paragraphs describing how these properties differ and how electronegativity is related to electron affinity and ionization energy.
Write in your notebook, then left-click here for an explanation.
Electronegativity is attraction for electrons in a bond by each bonded atom; it is an arbitrary scale with no units. Ionization energy and electron affinity are properties of individual atoms; each can be measured experimentally and is typically expressed in kJ/mol. Because of this, ionization energy and electron affinity are not the same as electronegativity.
That said, all three properties refer to attraction of an atom for an electron (or for electron density). Ionization energy measures the attraction of an atom for an electron that is part of the atom. Electron affinity measures the attraction of an atom for an electron that is not part of the atom. Electronegativity measures the attraction of an atom for shared electron density in a bond. Thus, we might expect elements with large ionization energies and large electron affinities (large negative EA values) to also have large electronegativites. (You can verify in the periodic table above that elements like fluorine, oxygen, and chlorine, which have large ionization energies and large negative electron affinities also are high on the electronegativity scale.)
Some theoretical chemists have defined electronegativity as the average of first ionization energy and the negative of electron affinity, that is, EN = (IE – EA)/2. Values calculated from this equation are then scaled to the range from 0 to 4 of the typical electronegativity scale. The resulting values are very similar to those in the periodic table above.
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