Electron Affinity Trend and Definition

Electron affinity (E_ea) is the energy change when a neutral atom in the gas phase gains an electron. 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.

Key Takeaways: Electron Affinity

• Electron affinity is the energy change when a neutral gas-phase 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.
• Chlorine has the highest known electron affinity; nobelium has the lowest.

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 (E_ea)!

E_ea = 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 E_ea 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 E_ea has a negative sign.

If the attachment of an electron releases 200 kJ/mol, then ΔE = –200 kJ/mol and Eea = +200 kJ/mol.

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 E_ea is positive. For the noble gases, ΔE is positive and E_ea 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 E_ea 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), E_ea actually increases as you move down the periodic table.

What Affects Electron Affinity

Several atomic factors influence the magnitude and sign of an element’s electron affinity. These factors help explain both general trends and exceptions across the periodic table:

1. Atomic Radius

As atomic radius increases, the added electron is farther from the nucleus and feels a weaker electrostatic attraction. This typically results in a lower electron affinity. Smaller atoms, like those in the upper-right corner of the periodic table (excluding noble gases), tend to have higher electron affinity values.

2. Nuclear Charge

The greater the effective nuclear charge (the net positive charge felt by the outermost electrons), the stronger the attraction between the nucleus and an added electron. This leads to more exothermic electron affinity. Elements across a period show increasing nuclear charge, contributing to the increase in electron affinity from left to right.

3. Electron Shielding

Inner electrons shield outer electrons from the full pull of the nucleus. In atoms with more electron shells, shielding reduces the effective nuclear attraction for additional electrons, lowering the electron affinity.

4. Subshell Configuration

Atoms with half-filled or fully filled subshells tend to resist gaining an extra electron due to increased electron–electron repulsion and orbital stability. This explains anomalies such as nitrogen having a lower electron affinity than expected.

5. Electron–Electron Repulsion

When an extra electron enters a compact or crowded orbital (especially in smaller atoms like fluorine), repulsion between electrons reduces the released energy, even though the nucleus has a strong pull. This is one reason why chlorine has a higher electron affinity than fluorine, despite fluorine’s higher electronegativity.

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.

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.

Applications of Electron Affinity

Electron affinity is more than just a periodic trend. It plays a key role in understanding chemical reactivity, bonding, and material behavior across many scientific fields:

1. Predicting Reactivity

• Elements with high electron affinity (like halogens) readily gain electrons to form stable anions.
• Elements with low or negative electron affinity (like noble gases and some metals) resist electron gain and often form cations instead.

2. Explaining Ionic Bond Formation

• Electron affinity helps determine how favorable it is for an atom to form an anion.
• For example, the formation of NaCl involves sodium losing an electron (low ionization energy) and chlorine gaining one (high electron affinity), forming a stable ionic compound.

3. Designing Semiconductors and Photovoltaics

• In materials science, the electron affinity of a material affects its band structure and work function, influencing how electrons move across junctions in semiconductors, LEDs, and solar cells.

4. Organic and Inorganic Chemistry

• Electron affinity values help chemists predict the behavior of radicals, anions, and reaction intermediates in various chemical pathways.

5. Astrophysics and Plasma Physics

• Understanding the electron affinity of atoms and ions helps model ionization and recombination processes in stars and interstellar environments.

FAQs About Electron Affinity

Why is electron affinity sometimes negative?

A negative electron affinity means that energy must be added to force an electron onto a neutral atom. This process is endothermic. It typically occurs with noble gases or atoms with full or stable subshells that resist accepting another electron.

Why does chlorine have a higher electron affinity than fluorine?

Although fluorine is more electronegative, its small size leads to significant electron–electron repulsion when adding an extra electron. Chlorine, being slightly larger, accommodates the additional electron more easily, releasing more energy in the process.

How is electron affinity different from electronegativity?

Electron affinity measures the energy change when an atom gains an electron in the gas phase, while electronegativity measures an atom’s tendency to attract electrons in a bond. Electron affinity is a quantitative property with units (kJ/mol or eV), whereas electronegativity is unitless and based on scales like the Pauling scale.

Do molecules have electron affinities?

Yes. Molecules can accept electrons, and this energy change is also called electron affinity. The value sometimes differs from atomic Eea due to molecular orbitals and relaxation effects. Molecular electron affinity is important in redox chemistry and mass spectrometry.

Is electron affinity always exothermic?

No. While many atoms release energy when gaining an electron (exothermic), some atoms require an input of energy (endothermic) for electron attachment, especially those with filled valence shells or stable subshell configurations.

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.