Electron Affinity in IB Chemistry: Definition and Trend

Electron affinity is an IB Chemistry term that students often mix up with electronegativity. The safest definition is: electron affinity is the enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous negative ions.

That wording matters because exam questions may reward the gaseous state, the electron gain, and the energy change.

Quick Answer

What Electron Affinity Means

Electron affinity is about adding an electron to a gaseous atom. It is not about attraction for a shared pair of electrons in a bond. That second idea is electronegativity.

The general equation for first electron affinity is:

X(g) + e- -> X-(g)

Use gaseous state symbols because electron affinity is defined for isolated gaseous atoms and ions.

Electron Affinity vs Electronegativity vs Ionization Energy

These three terms all involve electrons, but they test different ideas.

If the question mentions a covalent bond, think electronegativity. If it mentions removing an electron, think ionization energy. If it mentions gaining an electron, think electron affinity.

Periodic Trend in Electron Affinity

Across a period, first electron affinity usually becomes more exothermic because nuclear charge increases and atoms can attract an incoming electron more strongly. Students often describe this as becoming more negative.

Down a group, electron affinity generally becomes less exothermic because the incoming electron is added further from the nucleus and experiences more shielding. Electron affinity has exceptions, so avoid writing trend sentences as if there are no irregular values.

Worked Example

Write a definition of first electron affinity.

A mark-worthy answer is: first electron affinity is the enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous negative ions.

The important parts are one mole, gaseous atoms, electron gain, gaseous negative ions and enthalpy change.

Why Second Electron Affinity Is Different

Second electron affinity means adding an electron to an ion that is already negative. That is usually endothermic because the incoming electron is repelled by the negative ion.

For example, adding an electron to X-(g) to form X2-(g) requires energy input. The exam point is not just "second means two electrons"; it is that electron-electron repulsion makes the second addition harder.

Common Mistakes

Markscheme-Style Wording

For definitions, include these pieces:

• enthalpy change
• one mole of gaseous atoms
• one mole of electrons gained
• one mole of gaseous negative ions formed

That wording is safer than saying "how much an atom wants an electron".

Mini Practice Set

1. Write an equation for the first electron affinity of chlorine.
2. Explain why electron affinity is not the same as electronegativity.
3. State one reason periodic trends in electron affinity can have exceptions.
4. Explain why second electron affinity is usually endothermic.

Practice This Topic

Try this exam-style question:
Define first electron affinity and explain why it is different from electronegativity.

Answer guide:

• First electron affinity is an enthalpy change for gaseous atoms gaining electrons.
• Use the idea of forming gaseous negative ions.
• Electronegativity is attraction for a bonding pair of electrons.
• Do not describe electron affinity as attraction inside a covalent bond.

Practice this exact topic
Practice periodicity and electron affinity questions
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Related Study Links

• Electronegativity Trend Across a Period: IB Chemistry Guide
• Inert Gas vs Noble Gas: IB Chemistry Group 18 Guide
• Vaporization in IB Chemistry: Definition and Energy

FAQ

What is electron affinity in IB Chemistry?

Electron affinity is the enthalpy change when gaseous atoms gain electrons to form gaseous negative ions. For first electron affinity, one mole of gaseous atoms gains one mole of electrons. The definition is wording-heavy, so include gaseous state and negative ion formation. A vague phrase such as "attraction for electrons" is not precise enough.

Is electron affinity the same as electronegativity?

No, electron affinity and electronegativity are different. Electron affinity is an enthalpy change when an atom gains an electron. Electronegativity is the ability of an atom in a bond to attract the shared pair of electrons. Mixing them up is a common periodicity mistake. Check whether the question is about isolated atoms or bonded atoms.

How is electron affinity different from ionization energy?

Electron affinity involves gaining an electron, while ionization energy involves removing an electron. Electron affinity forms a negative ion from a gaseous atom. Ionization energy forms a positive ion from a gaseous atom. Check whether the question says gain or remove. That single verb often tells you which definition the examiner wants.

Why is second electron affinity endothermic?

Second electron affinity is usually endothermic because an electron is being added to an ion that is already negative. The incoming electron is repelled by the negative ion, so energy must be supplied. This is different from many first electron affinities, which are often exothermic. Mention repulsion if the question asks why energy is needed.

What should I include in an electron affinity definition?

Include enthalpy change, one mole of gaseous atoms, one mole of electrons gained, and one mole of gaseous negative ions formed. If the question asks for an equation, write X(g) + e- -> X-(g), then connect it to the definition. State symbols are useful because they show the gaseous condition clearly.

Final Takeaway

For electron affinity, keep the definition separate from electronegativity and ionization energy. If your answer says enthalpy change, gaseous atoms and negative ions, you are much closer to markscheme wording.