Electron Affinity Trends

Electron affinity is not a museum-piece atomic-structure number; it directly sets the oxidizing power of the halogens (fluorine and chlorine's large, favorable EA is part of why $\text{F}_2$ and $\text{Cl}_2$ are such aggressive oxidizers, alongside their weak X–X bonds), and it is one of the two raw ingredients (with ionization energy) feeding the Mulliken definition of electronegativity, $\chi_M = \tfrac{1}{2}(IE + EA)$, the bridge this article leads directly toward in Electronegativity Scales. In semiconductor physics, "electron affinity" is redefined as the energy from the conduction-band minimum to the vacuum level, and it governs band alignment at heterojunctions (the Anderson rule), Schottky barrier heights at metal-semiconductor contacts, and the design of photocathodes — negative-electron-affinity GaAs photocathodes (achieved by cesium/oxide surface treatment that pulls the vacuum level below the conduction band) are the enabling technology behind high-brightness electron sources used in electron microscopes and polarized-electron accelerators. In battery and energy-storage chemistry, the electron affinity of oxidized species (e.g., in lithium-ion cathode materials, or the oxygen/oxide couples in metal-air batteries) sets, together with lattice and solvation energetics, the achievable cell voltage; halogen-based redox shuttles in flow batteries are chosen in part on electron affinity and reduction-potential grounds. In atmospheric and astrophysical chemistry, the small but nonzero electron affinity of atomic hydrogen (72.8 kJ/mol) and of species like atomic oxygen governs negative-ion abundances in interstellar plasma and in the ionosphere, feeding models of radiative attachment rates. In synthetic and mechanistic organic/inorganic chemistry, electron affinities of aromatic and quinone-type molecules (a molecular, not atomic, extension of the same idea) are routinely used to rank oxidizing strength of electron-transfer mediators and to predict single-electron-transfer (SET) versus concerted mechanisms in redox catalysis.

Ask the tutor: Why doesn't the group-15 electron-affinity dip appear at all in ionization-energy trends for the same elements?

Start from the electron configuration of the neutral atom and ask: where does the next electron go, and what does it feel once it's there? Two forces act on it. First, it feels the attraction of the effective nuclear charge $Z_{\text{eff}}$, computed by Slater's rules exactly as for any other valence electron — this is the part of the story shared with Effective Nuclear Charge and with ionization energy. Second, it feels the repulsion of every electron already occupying that subshell (and, more weakly, other shells). Because the incoming electron enters the same principal shell that is being filled, this repulsion is large and shell-specific in a way that ionization-energy trends, which are dominated by the electron being removed from an already-populated shell, do not fully mirror. Consider period 2, filling the 2p subshell: B ($2p^1$), C ($2p^2$), N ($2p^3$), O ($2p^4$), F ($2p^5$), Ne ($2p^6$, closed). Up to nitrogen, each added electron goes into a different empty 2p orbital (Hund's rule keeps them unpaired, in B → C → N), so repulsion stays low and $Z_{\text{eff}}$ dominates: EA becomes more favorable from B to N... except that N already has three unpaired electrons in three different 2p orbitals, so the next electron added to make N⁻ must pair up in an already-singly-occupied orbital. That pairing costs exchange energy that is not recovered by the modest increase in $Z_{\text{eff}}$, so nitrogen's EA is anomalously small (slightly unfavorable, EA ≈ +7 kJ/mol by most tabulations, meaning N⁻ is not stable in the gas phase). Oxygen and fluorine, despite also having to pair electrons, are pulled in hard enough by the substantially larger $Z_{\text{eff}}$ that their EA is comfortably negative (favorable) — O: −141 kJ/mol, F: −328 kJ/mol. Down a group, radius grows and $Z_{\text{eff}}$ felt by the frontier subshell rises only slowly (shielding from inner shells nearly cancels the added protons), so naively EA should just decrease smoothly in magnitude. It does — mostly. But the second-row-to-third-row step (F → Cl, O → S, N → P) is anomalous: the second-row element's EA is often smaller in magnitude than the third-row congener's, because the compact 2p subshell of F, O, N packs the extra electron into unusually intense repulsion with the existing four electrons, a penalty that a size-doubled 3p subshell in Cl, S, P does not pay nearly as much. This is the same "second-row anomaly" that shows up in ionic radii and bond energies.

The picture above already contains the two central irregularities you must be able to explain from first principles: the dip at group 15 (N, P — pairing penalty in a half-filled subshell) and the crossover so that Cl beats F even though F is more electronegative and higher up the group (2p compactness penalty). Everything else — Ne and Ar sitting at essentially zero, the smooth rise B→C→O→F — follows directly from $Z_{\text{eff}}$ and radius, exactly the machinery used for ionization energy.

The defining thermodynamic quantity is the first electron affinity, defined operationally through the enthalpy of electron attachment:

where:

• $X(g)$ is a gas-phase neutral atom, $X^-(g)$ its singly-charged anion,
• $\Delta H_{\text{EA}}$ is the enthalpy change for electron attachment (negative = exothermic = favorable),
• "EA" as usually tabulated in general-chemistry tables is defined as $\text{EA} \equiv -\Delta H_{\text{EA}}$, so that a large positive tabulated EA (e.g. Cl, 349 kJ/mol) means a strongly exothermic, favorable process; this is the IUPAC-recommended "energy released" convention, and it is opposite in sign to the "$\Delta H$"-style convention used in some textbooks — always check which convention a source uses before comparing numbers.

Reading: this equation is nothing but Hess's-law bookkeeping, but the sign convention is the single most common place students lose marks, because unlike ionization energy (universally reported as a positive "energy required" number, no ambiguity), electron affinity is reported both ways in the literature. This article uses the IUPAC energy-released convention: more positive tabulated EA = more favorable electron capture, exactly parallel to bond dissociation energy language. The underlying microscopic model, exactly as for ionization energy, is a hydrogenic orbital energy corrected by effective nuclear charge:

where:

• $E_n$ is the (approximate) one-electron energy of the frontier orbital receiving the new electron,
• $Z_{\text{eff}} = Z - \sigma$ is the effective nuclear charge felt by that electron (Slater's rules give $\sigma$),
• $n$ is the principal quantum number of that orbital.

Reading: EA should track $Z_{\text{eff}}^2/n^2$ the same way ionization energy does — this explains the broad periodic increase across a row and decrease down a group. But this formula treats the target subshell as if it were a single electron in an otherwise-empty atom; it contains no term for the repulsion between the incoming electron and the electrons already present, which is exactly the term needed to explain the group-15 dip and the second-row anomaly. We must add it explicitly. A more complete energetic balance for electron attachment is:

where:

• the first term is the favorable orbital-energy contribution from nuclear attraction (grows across a period, shrinks down a group),
• $\sum_i J_{i,\text{new}}$ is the sum of Coulomb repulsion integrals between the new electron and each electron already occupying the subshell (or nearby shells) — this term grows sharply once the new electron must be placed in an orbital that already holds one electron (forced pairing),
• $K_{\text{exch}}$ is the favorable exchange energy recovered only between electrons of parallel spin in degenerate orbitals — it is lost, not gained, when the new electron must pair up.

Reading: this is the same competitive balance that governs Hund's-rule ordering in transition-metal ions and the stability of half-filled/filled subshells generally. When the incoming electron can occupy an empty orbital of the same subshell (as in B→C, C→N), $J$ stays small and EA rises steadily with $Z_{\text{eff}}$. The moment the subshell is half-full and the next electron must pair (N→N⁻, P→P⁻), $J$ jumps and $K_{\text{exch}}$ is lost, producing the anomalous dip visible in the figure above.

Example 1 (foundational) — Ranking three atoms by expected EA sign and magnitude. Rank Mg, Cl, and Ar by electron affinity (most negative $\Delta H_{\text{EA}}$, i.e. most favorable electron capture, to least favorable / most positive), and justify each qualitatively. Reasoning. Mg is $[Ne]3s^2$: a filled 3s subshell. The next electron must enter the 3p subshell, higher in energy and poorly penetrating, and Mg's $Z_{\text{eff}}$ for a 3p electron is low because the filled 3s² provides no extra shielding advantage — the process is endothermic, EA(Mg) ≈ −40 kJ/mol on the IUPAC convention (i.e. $\Delta H_{\text{EA}} \approx +40$ kJ/mol, unfavorable). Cl is $[Ne]3s^23p^5$: one empty slot in the 3p subshell (by orbital-filling, one 3p orbital has just one electron paired against the incoming one, or more precisely 3p⁵ has the pattern ↑↓ ↑↓ ↑, so the sixth electron pairs the lone unpaired one) and a high $Z_{\text{eff}}$ (6.10, from the derivation above); this is the most exothermic case, EA(Cl) = +349 kJ/mol (favorable). Ar is $[Ne]3s^23p^6$: completely filled valence shell; the next electron must start a brand-new 4s subshell, far outside the compact, well-shielded core, so the process is strongly endothermic, EA(Ar) ≈ −77 kJ/mol (unfavorable, $\Delta H_{\text{EA}} \approx +77$ kJ/mol). Ranking (favorable → unfavorable): Cl (+349) > Mg (−40) > Ar (−77). Notice Mg is less unfavorable than Ar, even though both refuse the extra electron: Mg⁻ merely has to start populating an existing, already-partly-relevant subshell type (3p) in the same shell, whereas Ar⁻ must jump to an entirely new principal shell (n=4), a bigger energetic leap. This illustrates that "closed subshell → unfavorable EA" is not a single fixed penalty; the size of the penalty depends on how far away the next available orbital is.

Example 2 (applied) — Estimating a lattice-energy-relevant quantity: which halide gives the more exothermic overall "atom + electron" step in forming an ionic salt. A materials chemist is comparing NaF and NaCl formation via the Born–Haber cycle and wants to know how much of the difference in lattice-formation favorability (before lattice energy itself is considered) comes purely from the electron-affinity step, given EA(F) = 328 kJ/mol and EA(Cl) = 349 kJ/mol (IUPAC convention, energy released). Reasoning. The relevant Born–Haber step is $\text{X}(g) + e^- \to \text{X}^-(g)$ with enthalpy $\Delta H_{\text{EA}} = -\text{EA}$. For F, $\Delta H_{\text{EA}} = -328$ kJ/mol; for Cl, $\Delta H_{\text{EA}} = -349$ kJ/mol. The difference is

So purely from the electron-affinity step, forming Cl⁻ releases 21 kJ/mol more energy than forming F⁻ — the opposite of what naive "F is smaller and more electronegative, so it should bind an electron harder" reasoning would predict, exactly the second-row anomaly quantified above. This 21 kJ/mol is a small piece of the full Born–Haber cycle (lattice energies for these salts are on the order of 900 (NaCl) to 920 (NaF) kJ/mol in magnitude, and it is actually the smaller ionic radius of F⁻ that dominates in making NaF's lattice energy larger and the compound overall more stable, not electron affinity). The exercise shows explicitly that electron affinity, while directionally important, is often not the dominant term controlling overall ionic-compound stability — lattice energy (governed by ionic radius, via Effective Nuclear Charge and shielding arguments) usually swamps it. Limiting-case check. If EA(F) had (counterfactually) exceeded EA(Cl), as the naive $Z_{\text{eff}}$-only picture predicts, the difference above would flip sign; the fact that real thermochemical data gives $-21$ kJ/mol, not $+21$ kJ/mol, is the experimental confirmation of the repulsion-based mechanism described in the second derivation.

Example 3 (exam / competition level) — Predicting and explaining a specific numeric anomaly: O vs S vs Se electron affinities. Given EA(O) = 141 kJ/mol, EA(S) = 200 kJ/mol, EA(Se) = 195 kJ/mol (all IUPAC convention), (a) identify which of these three breaks the simple "EA decreases down a group" rule, and (b) explain the break using the same competing-term framework used above, then (c) predict, with reasoning, whether EA(Te) should continue the S→Se trend or reverse again. Reasoning (a). Going O → S, EA increases (141 → 200 kJ/mol) — this itself looks like an "anomaly" relative to the textbook-simple statement "EA decreases down a group," and is in fact the same second-row effect as F vs Cl: O's 2p subshell is so compact that electron-electron repulsion partially offsets its $Z_{\text{eff}}$ advantage, exactly as for F vs Cl. Going S → Se, EA decreases slightly (200 → 195 kJ/mol), which does follow the simple $Z_{\text{eff}}/n$ argument — from period 3 downward, radius grows enough and shielding is efficient enough that the naive picture reasserts itself, exactly as seen for Cl → Br → I in the earlier limiting-case check. Reasoning (b). Using the balance equation, the O → S step has ΔJ (repulsion drop from shrinking-subshell relief) dominating over Δ(nuclear attraction term), for the same reason as F → Cl: O's 2p subshell is unusually crowded for its size. The S → Se step has both terms shrinking together in the "normal" way — $Z_{\text{eff}}$ for the frontier electron rises only slightly (poor shielding by filled $d^{10}$ in Se partly compensates, but not enough to reverse the trend), while $n$ increases from 3 to 4, so the orbital-energy term $Z_{\text{eff}}^2/n^2$ genuinely falls, and there is no compensating repulsion collapse large enough to reverse it (S's 3p subshell is not "anomalously crowded" the way O's 2p is). Reasoning (c) — prediction for Te. Following the well-behaved period 4→5 pattern (analogous to Cl → Br → I decreasing smoothly), EA(Te) should continue the modest decline from Se, and indeed the experimental value, EA(Te) ≈ 190 kJ/mol, confirms a small further decrease, with no second anomaly expected because the "compact second-row subshell" effect is a one-time phenomenon confined to period 2 (and, to a much smaller extent, period 3 relative to period 4); by period 5 the radial expansion is smooth and monotonic behavior resumes, just as it does for the halogens beyond Cl.

The systematic ranking of electron affinities as a periodic property owes most to spectroscopists and physical chemists of the mid-20th century who measured them via photodetachment and thermochemical cycles, since electron affinity — unlike ionization energy — cannot be measured by simple discharge spectroscopy of a neutral atom; you need a stable, isolable anion or a photoelectron-spectroscopy experiment on a gas-phase anion beam.