Electron affinity
Electron affinity asks a narrow but revealing question: what energy change accompanies attaching an electron to a neutral gas-phase atom? Nuclear attraction favors attachment; shielding, orbital size and electron–electron repulsion push back. The competition produces a periodic pattern with meaningful exceptions.
Definition and periodic pattern
Definition and sign convention
The first electron-attachment process is:
X(g) + e⁻ → X⁻(g)
Some tables report the energy released as a positive value; thermochemical tables often report an enthalpy change ΔH that is negative for an exothermic attachment. A value is meaningless unless the sign convention is known.
Electron affinity is measured or inferred for an isolated gas-phase atom. Electronegativity is different: it describes attraction for electron density while an atom is bonded.
Periodic trend and exceptions
Across much of a period, rising effective nuclear charge makes electron attachment more favorable. Down a group, larger orbitals and greater shielding usually weaken the attraction.
Group 2 atoms, group 15 atoms and noble gases break a smooth trend because their starting subshell configurations are especially stable. The incoming electron may have to enter a new subshell or pair with an electron already present.
Fluorine versus chlorine
Fluorine attracts bonding electrons more strongly than chlorine, yet chlorine has the more favorable first electron affinity. The added electron in F enters a compact 2p region where electron–electron repulsion is unusually strong. Chlorine’s larger 3p region gives the new electron more room.
This is a useful warning against one-factor explanations: stronger nuclear attraction can be offset by stronger crowding.
From isolated atoms to ionic solids
First and second attachments
A second electron must be added to an already negative ion:
X⁻(g) + e⁻ → X²⁻(g)
That gas-phase step is always energetically unfavorable because the incoming electron is repelled by the negative ion. O²⁻ and S²⁻ can still be stable in ionic solids because the crystal lattice provides strong electrostatic stabilization.
Born–Haber cycles and ionic solids
Formation of an ionic solid can be decomposed into atomization, ionization, electron attachment and lattice formation. Electron affinity is only one term in this energy accounting.
A favorable electron affinity therefore does not, by itself, prove that a compound will form. Ionization energies, lattice energy, stoichiometry and competing structures all contribute to the total free-energy balance.
Measurement and interpretation
What the quantity does and does not tell us
Useful
Comparing isolated atoms that accept one electron; explaining halogen behavior; checking periodic trends; Born–Haber cycles.
Not enough by itself
Predicting complete reactivity, bond type, aqueous stability or a preferred oxidation state.
Ionization energy, electron affinity and electronegativity arise from related electron structure, but they answer different questions.
How electron affinity is obtained experimentally
Electron affinity is difficult to measure by simply “watching” a free electron attach to a neutral atom. A common route is to study the reverse process: photodetachment. Light removes an electron from a negative ion, and the threshold photon energy reveals the binding energy of that extra electron.
This reverse-process view is useful conceptually. If X⁻ requires energy to lose its extra electron, the neutral atom X releases the corresponding energy when that electron attaches, once the sign convention is handled consistently.
Gas-phase measurements matter because solvents and crystals add new stabilization. The electron affinity of an isolated chlorine atom is therefore not the same thermodynamic quantity as the free energy for forming Cl⁻ in water.
Numbers make the exceptions concrete
Representative first electron affinities illustrate the structure behind the trend. Chlorine releases about 3.6 eV when it gains an electron, fluorine about 3.3 eV, while nitrogen is much less favorable because of its half-filled 2p subshell. Noble-gas attachment is unfavorable because the new electron must enter a higher shell.
The difference between F and Cl is not a failure of periodicity. It is periodicity plus electron repulsion: fluorine’s compact valence shell increases both nuclear attraction and crowding. The observed value is the balance of those effects.
Electron affinity also should not be turned directly into a redox potential. Electrode potentials include solvation, molecular bond breaking, ion pairing and other free-energy terms. Atomic electron affinity is one microscopic contribution, not the whole electrochemical reaction.
Energy units and a useful scale comparison
Electron affinities are often quoted either in electronvolts per atom or in kilojoules per mole. The two descriptions represent the same energy scale: 1 eV per particle is about 96.5 kJ mol⁻¹. This conversion helps connect atomic measurements to thermochemical calculations.
For chlorine, an attachment energy near 3.6 eV corresponds to roughly 350 kJ mol⁻¹ released per mole of atoms. That is large enough to matter strongly in chemistry, but it is still only one term among ionization, bond breaking, solvation and lattice formation.
Using both units also prevents a common mistake: an electron affinity is an energy change for one electron-attachment event, not a dimensionless score of how much an atom “wants” electrons.
Using electron affinity
Reading a period without memorizing a zigzag
Period 3 shows how the competing effects build the pattern. Na has one 3s electron, so adding another electron is modestly favorable. Mg already has 3s²; the next electron must enter 3p, making attachment much less favorable. Al begins filling 3p and becomes more favorable again.
From Si toward Cl, increasing effective nuclear charge generally favors attachment, but P and S reveal the effect of occupancy. P has a half-filled 3p³ arrangement; the next electron must pair. S has already begun pairing, so the balance changes again. Cl is strongly favorable because adding one electron completes 3p⁶.
Ar is different: its valence shell is already closed. An additional electron would have to occupy the next principal shell, much farther from the nucleus and strongly shielded. The trend therefore ends with a sharp change rather than a smooth continuation.
Connection to ions, oxidation and reduction
Electron affinity helps explain why halogen atoms readily form X⁻ ions in many reactions, but a chemical reduction is rarely just an isolated atom capturing a free electron. Real reactions may break bonds, reorganize solvent molecules, change lattice energy and involve several electrons at once.
That distinction matters in electrochemistry. A standard reduction potential is a free-energy quantity for a complete half-reaction under specified conditions. It cannot be predicted from atomic electron affinity alone, although both reflect how electron structure influences electron gain.
Use electron affinity as a microscopic building block: it tells you about the gas-phase atom. Then add the rest of the thermodynamic environment before predicting a real chemical process.
Exercises
F or Cl?
Which has the more favorable first electron affinity, F or Cl?
Solution
Cl. Fluorine’s compact 2p region creates unusually strong electron–electron repulsion for the incoming electron.
Nitrogen
Why does N deviate from a smooth across-period trend?
Solution
N has a half-filled 2p³ subshell. The added electron must pair in an occupied p orbital, increasing repulsion.
Oxide
How can O²⁻ be common in crystals if the second gas-phase electron attachment is unfavorable?
Solution
The ionic lattice strongly stabilizes O²⁻ through attractions to cations, compensating for the unfavorable gas-phase step.