Electron Affinity Trends — Chemistry Mnemonic
Target: mnemonic for electron affinity trends periodic table
Why is this hard to memorize?
Electron affinity (EA) is the energy change when an electron is added to a gaseous atom. Chlorine has the highest EA (−349 kJ/mol), not Fluorine — this is a classic exam trick. The general trends: EA becomes more negative (more energy released) across a period and less negative down a group. Exceptions are key: Noble gases have positive EA (don't want electrons), Be and N have low/positive EA (full/half-filled subshells), and F has lower EA than Cl (small size causes electron-electron repulsion). NEET and JEE love the F vs Cl comparison.
Classic mnemonics you should know
"Cl > F in EA — "Chlorine is the Champion of electron affinity, not Fluorine""
Despite F being more electronegative, Cl has the highest electron affinity. Fluorine is too small — adding an electron to its compact 2p orbital causes severe electron-electron repulsion, reducing the energy released. Chlorine's larger 3p orbital accommodates the extra electron comfortably.
"Be(s²), N(p³), Ne(p⁶) all have low/positive EA — fully or half-filled subshells don't want more electrons"
Same stability rule as IE exceptions: filled or half-filled subshells resist adding electrons. Be(2s²) → positive EA. N(2p³) → much less negative than C. Noble gases → positive EA.
"Across: more negative (like IE). Down: less negative (like IE). But always remember Cl > F."
EA trends are similar to IE trends but with more exceptions. The key exam fact: Cl has the most negative EA of all elements, followed by F, then Br.
The complete list
- EA becomes more negative across period
- EA becomes less negative down group
- Cl has highest EA (−349 kJ/mol)
- F has lower EA than Cl (repulsion)
- Noble gases: positive EA
- Be, N: low EA (stable subshell)
- Second EA is always positive (repulsion)
Frequently asked questions
Why does Chlorine have higher electron affinity than Fluorine?
Fluorine's 2p orbital is very small. Adding a 7th electron to this tiny space causes strong electron-electron repulsion, reducing the energy released. Chlorine's 3p orbital is larger, so the added electron fits comfortably with less repulsion. Hence Cl releases MORE energy when gaining an electron (EA of Cl = −349 kJ/mol vs F = −328 kJ/mol).
What is the difference between electron affinity and electronegativity?
Electron affinity is a measurable energy value for an isolated gaseous atom gaining one electron. Electronegativity is a relative scale measuring an atom's ability to attract shared electrons in a BOND. They follow similar trends but are different properties. F is most electronegative; Cl has highest electron affinity.
Why is the second electron affinity always positive?
Adding a first electron to a neutral atom can be favorable (negative EA). But adding a second electron means pushing a negative electron into an already negative ion (X⁻). The electron-electron repulsion requires energy INPUT, making the second EA always positive. This is why O²⁻ and S²⁻ don't exist in isolation — they only exist stabilized in crystal lattices.
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