What is Electron Affinity?
Electron affinity is the energy change (usually released) when an electron is added to a neutral atom in the gas phase. It measures how much an atom wants to gain an electron — the opposite of ionization energy. Atoms with high electron affinity (like halogens) readily accept electrons and are strong oxidizing agents.
Electron affinity (EA) is the energy released when one electron is added to a neutral atom: A(g) + e⁻ → A⁻(g). Usually negative (energy released); higher magnitude means stronger attraction to electrons. Halogens have the highest EA; noble gases have very low EA.
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Step-by-step worked examples
Fluorine (F) has electron affinity of 328 kJ/mol. What does this mean?
F + e⁻ → F⁻ releases 328 kJ/mol of energy. Fluorine strongly attracts electrons — it's the most electronegative element. This high EA makes F a powerful oxidizing agent.
Compare electron affinity: Cl (349 kJ/mol) vs Br (325 kJ/mol). Why is Cl higher?
Cl is smaller than Br (period 3 vs period 4). The added electron is closer to the nucleus in Cl. Closer attraction → higher energy release → higher EA magnitude.
Noble gas (He) has EA of −48 kJ/mol (negative!). What does negative mean?
Energy must be SUPPLIED to add an electron to He. He is stable with full 1s² shell — adding an electron is unfavorable. Negative EA means the atom resists accepting electrons.
Flashcards
Quick quiz
Q1.Electron affinity is defined as…
Q2.Which element has the highest electron affinity?
Q3.Noble gas electron affinity is positive because…
Q4.Why is Cl electron affinity (349 kJ/mol) higher than Br (325 kJ/mol)?
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Common mistakes
Electron affinity is the same as electronegativity. — Correct: EA is the energy when an ISOLATED atom gains an electron. Electronegativity is the ability to attract electrons IN A BOND.
Positive electron affinity means high attraction to electrons. — Correct: Negative EA = favorable, released energy. Positive EA = unfavorable, requires energy input (rare, noble gases).
Noble gases have the highest electron affinity. — Correct: Noble gases have LOW (often positive) EA — adding an electron to a stable full shell is unfavorable.
All atoms have negative electron affinity values. — Correct: Most do, but noble gases have positive or near-zero EA — they don't want electrons.
FAQ
What is the formula for electron affinity?
No single formula, but EA can be estimated using: EA ≈ −13.6(Z − σ)² / n² eV, where Z is atomic number, σ is shielding, and n is shell.
Why do halogens have such high electron affinity?
Halogens need just one more electron to complete a stable octet. The nucleus is also small and highly attractive.
Can electron affinity be positive?
Yes, for noble gases and some transition metals. Positive EA means energy must be supplied (unfavorable).
How do EA and ionization energy relate?
They are opposite: high IE (hard to remove electron) ≠ high EA (willing to add electron). E.g., noble gases have high IE but low EA.