Electron affinity (Eea) is the energy change when an electron is added to a neutral atom in the gas phase. In simple terms, it is a measure of a neutral atom’s ability to gain an electron. The gas phase atom is used (rather than liquid or solid) because the atom’s energy levels aren’t influenced by neighboring atoms. The most common units for electron affinity are kilojoules per mole (kJ/mol) or electronvolts (eV). Electron affinity also applies to molecules, in some cases.
- Electron affinity is the energy change when an atom gains an electron.
- For most elements, except noble gases, this is an exothermic process.
- Electron affinity increases moving across a period and sometimes decreases moving down a group.
- The reason electron affinity increases moving across a period is because the effective nuclear charge increases, which attracts electrons.
History
In 1934, Robert S. Mulliken applied electron affinities to list an electronegativity scale for the atoms of the periodic table. Electronic chemical potential and chemical hardness also use the principle of electron affinity. An atom with a more positive electron affinity value than another atom is an electron acceptor, while one with a less positive value is an electron donor.
How Electron Affinity Works (Sign Convention)
Atoms gain or lose energy when they gain or lose electrons or participate in chemical reactions. The sign of the energy change depends on whether you attach or remove an electron. Use care, because the sign for the change in energy (ΔE) is the opposite of the sign for electron affinity (Eea)!
Eea = ΔE(attach)
For attaching an electron:
- When atoms release energy, the reaction is exothermic. The change in energy ΔE has a negative sign and the electron affinity Eea has a positive sign.
- When atoms absorb energy, the reaction is endothermic. The change in energy ΔE has a positive sign and the electron affinity Eea has a negative sign.
Electron affinity for most atoms on the periodic table, except the noble gases, is exothermic. Basically, energy is required in order to attach an electron. So, for most atoms, ΔE is negative and Eea is positive. For the noble gases, ΔE is positive and Eea is negative. A noble gas atom is already stable, so it absorbs energy to capture another electron. For noble gases, electron capture is endothermic.
However, some tables list values for the removal of an electron from a neutral atom rather than the capture of an electron. The energy value is equivalent, but the sign is reversed.
Electron Affinity Trend on the Periodic Table
Like electronegativity, ionization energy, atomic or ionic radius, and metallic character, electronegativity displays periodic table trends. Unlike some of these other properties, there are many exceptions to the trends for electron affinity.
- Electron affinity general increases moving across a row or period of the periodic table, until you reach group 18 or the noble gases. This is because of the filling of the valence electron shell moving across a period. For example, a group 17 (halogen) atom becomes more stable by gaining an electron, while a group 1 (alkali metal) must add several electrons to reach a stable valence shell. Further, the effective nuclear charge increases as you move across a period.
- Noble gases have low electron affinities.
- Generally (with exceptions) nonmetals have a higher or more positive Eea value than metals.
- Atoms that form anions that are more stable than the neutral atoms have high electron affinity values.
- Although usually depicted on a diagram of periodic table trends, electron affinity does not reliably decrease moving down a column or group. In group 2 (alkaline earth metals), Eea actually increases as you move down the periodic table.
Difference Between Electron Affinity and Electronegativity
Electron affinity and electronegativity are related concepts, but they don’t mean the same thing. In a way, both are a measure of an atom’s capacity to attract an electron. But, electron affinity is a gaseous neutral atom’s energy change upon accepting an electron, while electronegativity is a measure of how easily an atom attracts a bonding pair of electrons that can form a chemical bond. The two values have different units and somewhat different periodic table trends.
| Electronegativity | Electron Affinity | |
|---|---|---|
| Definition | Ability of atom to attract electron | Amount of energy released or absorbed when neutral atom or molecule accepts electron |
| Application | Single atom only | Usually, single atom, but the concept also applies to a molecule |
| Units | Pauling units | kJ/mol or eV |
| Property | Qualitative | Quantitative |
| Periodic Table Trend | Increases moving left to right across a period (except noble gases) Decreases moving down a group | Increases moving left to right across a period (except noble gases) |
Which Element Has the Highest Electron Affinity?
Halogens, in general, readily accept electrons and have high electron affinities. The element with the highest electron affinity is chlorine, with a value of 349 kJ/mole. Chlorine gains a stable octet when it captures an electron.
The reason why chlorine has a higher electron affinity than fluorine is because the fluorine atom is smaller. Chlorine has an additional electron shell, so its atom more easily accommodates the electron. In other words, there is less electron-electron repulsion in the chlorine electron shell.
Which Element Has the Lowest Electron Affinity?
Most metals have lower electron affinity values. Nobelium is the element with the lowest electron affinity (-223 kJ/mol). Nobelium atoms have an easy time losing electrons, but forcing another electron into an atom that’s already huge isn’t thermodynamically favorable. All of the existing electrons act as a screen against the positive charge of the atomic nucleus.
First Electron Affinity vs Second Electron Affinity
Usually, tables list the first electron affinity. This is the energy change of adding the first electron to a neutral atom. For most elements, this is an exothermic process. On the other hand, the energy change of adding a second electron is the second electron affinity value. Usually, this requires more energy than the atom gains. Most second electron affinity values reflect endothermic processes.
So, if the first electron affinity value is positive, then the second electron affinity value usually is negative. If you use the other sign convention, if the first electron affinity is negative, then the second electron affinity is positive.
References
- Anslyn, Eric V.; Dougherty, Dennis A. (2006). Modern Physical Organic Chemistry. University Science Books. ISBN 978-1-891389-31-3.
- IUPAC (1997). “Electron affinity.” Compendium of Chemical Terminology (the “Gold Book”) (2nd ed.). Oxford: Blackwell Scientific Publications. doi:10.1351/goldbook.E01977
- Mulliken, Robert S. (1934). “A New Electroaffinity Scale; Together With Data on Valence States and on Valence Ionization Potentials and Electron Affinities.” J. Chem. Phys. 2: 782. doi:10.1063/1.1749394
- Tro, Nivaldo J. (2008). Chemistry: A Molecular Approach (2nd Ed.). New Jersey: Pearson Prentice Hall. ISBN 0-13-100065-9.