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Ionization energy asks how hard it is to take an electron away from an atom. Electron affinity asks the reverse question: how much does an atom want an extra electron? It’s a key reason halogens form negative ions so readily — and it contains one of the best-known surprises on the periodic table.
The definition
The electron affinity of an element is the energy change when an electron is added to a neutral atom in the gas phase to form a negative ion:
X(g) + e⁻ → X⁻(g)
For most elements, adding an electron releases energy, because the incoming electron is attracted to the nucleus.
The sign convention (read this carefully)
Different sources use opposite signs, and it causes endless confusion:
- As a thermodynamic energy change, energy released is negative. Chlorine’s is about −349 kJ/mol.
- As an “affinity”, many tables (and our element data) report the energy released as a positive number. Chlorine’s is 3.617 eV (the same thing: 349 kJ/mol).
Either way, a larger magnitude means the atom holds on to the extra electron more strongly. Always check which convention a table is using.
The general trend
- Across a period → generally increases (more energy released). Atoms get smaller and effective nuclear charge rises, so an added electron is attracted more strongly.
- Down a group → generally decreases. The incoming electron sits further from the nucleus.
So, like electronegativity, electron affinity peaks towards the top right — the halogens have the highest values of all. You can see the pattern, gaps and all, on the electron affinity trend map.
The highest values (from our element data)
| Element | Electron affinity (eV) |
|---|---|
| Chlorine | 3.617 |
| Bromine | 3.365 |
| Fluorine | 3.339 |
| Iodine | 3.059 |
| Astatine | ~2.8 |
| Gold | 2.309 |
| Platinum | 2.128 |
| Sulfur | 2.077 |
Gold’s high value is striking for a metal. Relativistic effects stabilize its 6s orbital, and gold can even form the auride ion, Au⁻, in compounds like cesium auride (CsAu).
Why chlorine beats fluorine
Fluorine is the most electronegative element and the most reactive nonmetal, so you might expect it to have the highest electron affinity. It doesn’t — chlorine does.
The reason is size. Fluorine is a very small atom, and its 2p subshell is already crowded with seven electrons packed close together. An extra electron squeezed into that small space experiences strong repulsion from the electrons already there, which cancels some of the energy released. Chlorine’s 3p subshell is larger and roomier, so the added electron is repelled less.
The same thing happens with oxygen (1.461 eV) and sulfur (2.077 eV): the second-row element is lower than the one below it.
This doesn’t make chlorine more reactive than fluorine. Fluorine’s reactivity also depends on its very weak F–F bond, which is easy to break, and on the strong bonds it forms with other elements.
Elements that don’t want an extra electron
Some elements have electron affinities close to zero or negative (energy must be supplied to add an electron), so their anions are unstable. Our data leaves these as unknown (null), because a stable anion can’t be formed to measure:
- Noble gases — a new electron would have to start a new, higher shell.
- Beryllium and magnesium (and the group 2 elements generally) — their filled s subshells mean an added electron must go into a higher p subshell.
- Nitrogen — its half-filled 2p subshell is relatively stable, and adding an electron forces pairing.
These are the same subshell effects that cause the dips in ionization energy.
Second electron affinity
Adding a second electron to a negative ion — as in O⁻ → O²⁻ — always requires energy, because you’re pushing an electron onto something that’s already negative. So why do oxide ions (O²⁻) exist in compounds like MgO? Because the energy released when the ions pack together into a crystal lattice more than pays for it.
Quick answers
Which element has the highest electron affinity? Chlorine.
Is electron affinity the same as electronegativity? No. Electron affinity is a measured energy for an isolated gaseous atom; electronegativity describes an atom’s pull on shared electrons within a bond.
Why is electron affinity sometimes negative? Partly because of sign conventions, and partly because some atoms genuinely require energy to accept an electron.
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