Electron Affinity

Key Ideas

• Definition: the energy change for X(g) + e⁻ → X⁻(g).
• Sign convention (stated explicitly): In this note, EA is reported as a positive number equal to the energy released when the electron is added (the common table convention). The corresponding enthalpy change for the reaction is negative: ΔH = −EA. Some textbooks instead quote the negative ΔH directly — always check which convention a table uses.
• Exothermic (favorable) EA: energy is released; the atom "wants" the electron (typical of halogens and oxygen).
• Near-zero or positive (unfavorable) EA: little or no energy released, or energy must be supplied (Groups 2 and 15, and the noble gases).
• Across a period (→): EA generally becomes more favorable (larger) as Zeff rises.
• Down a group (↓): EA generally becomes less favorable (smaller) because the added electron enters a larger, more shielded shell.
• Second EA: X⁻(g) + e⁻ → X²⁻(g) is always endothermic — you must force a second electron onto an already-negative ion.

Equations and Variables

• X(g) + e⁻ → X⁻(g), ΔH = −EA (using the "energy released" convention).
  • EA = electron affinity, in kJ/mol (positive value = energy released).
  • ΔH = enthalpy change (negative when exothermic).
• First vs. second EA: EA₁ (neutral atom) is often exothermic; EA₂ (from a 1− anion) is always endothermic. Example for oxygen: EA₁ = +141 kJ/mol (released), but EA₂ ≈ −744 kJ/mol (i.e., ΔH = +744 kJ/mol must be supplied).

How It Works

1. An approaching electron is attracted by the nucleus (Zeff) but repelled by the existing electrons.
2. If attraction wins, energy is released and a stable anion forms — a favorable (large, exothermic) EA.
3. Across a period, rising Zeff strengthens the attraction, so EA becomes more favorable.
4. Down a group, the added electron enters a larger, more shielded shell, so attraction weakens and EA becomes less favorable.
5. Exceptions: Group 2 (filled s²) and Group 15 (half-filled p³) are already stable, so they show little or no energy release. Fluorine's 2p shell is so compact that the added electron suffers strong repulsion, making F's EA slightly less favorable than Cl's despite F's higher electronegativity.
6. Adding a second electron (to form a 2− ion) always costs energy because it must overcome the anion's negative charge — that is why O²⁻ and S²⁻ exist only when stabilized in a crystal lattice by nearby cations.

Worked Example

1. Which has the more favorable electron affinity, Cl or F? Chlorine (EA ≈ 349 kJ/mol released) is slightly more favorable than fluorine (EA ≈ 328 kJ/mol released). Even though F is smaller and more electronegative, its compact 2p shell crowds the incoming electron, so less energy is released than when chlorine's roomier 3p shell accepts an electron.

2. Why is the EA of nitrogen near zero? Nitrogen has a half-filled 2p³ configuration (one electron in each p orbital), which is relatively stable. Adding an electron forces pairing, which costs energy, so N releases almost no energy (EA ≈ 0). The same reasoning applies to phosphorus.

Common Confusions

• "Electron affinity is the energy to remove an electron." — Wrong: that is ionization energy. Electron affinity adds an electron.
• "A negative electron affinity always means unfavorable." — Depends on convention. In the "energy released" convention, a negative number means energy must be added (unfavorable). In the ΔH convention, negative means released. Always state the convention.
• "Fluorine has the most favorable EA because it's the most electronegative." — Wrong: chlorine's EA is larger than fluorine's (the F anomaly), even though F is more electronegative.
• "Groups 2 and 15 have large electron affinities." — Wrong: their filled s² and half-filled p³ shells are already stable, so their EAs are near zero.