Define first electron affinity
Describe the enthalpy change when gaseous atoms gain one mole of electrons to form gaseous 1− ions.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 23. Use the syllabus sign convention for lattice enthalpy (gaseous ions forming the ionic solid); state phases, signs, units and temperature. Written practice is self-checked
Before you begin
- Use AS enthalpy changes and Hess's law, formulae for ions, balanced equations, and temperature in kelvins.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which starting particle appears in the definition of first electron affinity?
What you will learn
- State the gaseous atom and gaseous ion states
- Define the one-mole basis for first electron affinity
- Relate the sign to electron gain
First electron affinity is a gas-phase process
The first electron affinity, EA₁, is the enthalpy change when one mole of gaseous atoms each gains one electron to form one mole of gaseous 1− ions. For chlorine: Cl(g) + e⁻ → Cl⁻(g).
An isolated atom usually attracts the added electron and releases energy, so first electron affinity is commonly negative. State the actual definition and equation; do not confuse it with ionisation energy, which removes an electron.
Keep first and later electron additions distinct
EA₁ adds an electron to a neutral gaseous atom. A later electron affinity would add an electron to an already negative ion, so electron–electron repulsion can make that step endothermic. Topic 23.1.2 asks for the first electron affinity and trends in it.
Compare values by saying more or less exothermic, or more or less negative. A numerically smaller negative value is less exothermic, not a larger energy release.
Pause and explain
What happens to the atom in the first electron-affinity process?
Worked example
Define the first electron affinity of chlorine and explain the sign of its enthalpy change.
Show the worked solution
- Begin with gaseous chlorine atoms.
- Add one electron per atom to form gaseous chloride ions.
- The attraction releases energy.
Answer Cl(g) + e⁻ → Cl⁻(g). EA₁ is the enthalpy change when one mole of gaseous Cl atoms gains one mole of electrons to form one mole of gaseous Cl⁻ ions; it is negative because energy is released.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write an equation for the first electron affinity of oxygen.
Give me a hint
Use gaseous atoms, one electron and gaseous 1− ions.
Compare my reasoning
- Start with O(g).
- Add e⁻ to the left-hand side.
- Form O⁻(g), not O²⁻(g).
O(g) + e⁻ → O⁻(g).
Look for these in your work
- Gaseous atom used
- One electron added
- First ion has charge 1−
A student defines first electron affinity as Cl⁻(g) → Cl(g) + e⁻. Correct the direction and describe what the student's equation represents.
Give me a hint
The definition is electron gain, while the proposed equation shows electron loss.
Compare my reasoning
- Reverse the electron-transfer direction.
- Use Cl(g) + e⁻ → Cl⁻(g).
- State that the proposed equation is the reverse process.
First electron affinity is Cl(g) + e⁻ → Cl⁻(g), per mole. The student's equation shows electron detachment, the reverse direction.
Look for these in your work
- Electron gain restored
- Gas states retained
- Reverse process identified
Explain why a first electron-affinity value is not an ionisation-energy value, even though both involve a gaseous atom or ion.
Give me a hint
Compare whether an electron enters or leaves the species.
Compare my reasoning
- EA₁ adds an electron to a gaseous atom.
- Ionisation removes an electron from a gaseous atom or ion.
- The electron-transfer direction and resulting charge differ.
First electron affinity is electron gain by a gaseous atom to form a 1− ion. Ionisation energy is energy required to remove an electron, producing a more positive ion; the processes have different directions and signs.
Look for these in your work
- Electron direction compared
- Products compared
- Sign distinction stated
Common mistakes
- Starting with Cl₂(g) rather than gaseous atoms
- Forming Cl⁻(aq) instead of Cl⁻(g)
- Describing electron removal
- Forgetting the one-mole basis
What can you explain now?
Which starting particle appears in the definition of first electron affinity?
Compare with the explanation
A gaseous atom
EA₁ adds an electron to a gaseous neutral atom.
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Make it stick
What species gains or loses an electron in first electron affinity compared with ionisation?
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