Electron affinity is closely tied to how strongly an atom attracts an incoming electron. In IB Chemistry Topic 2 (Atomic Structure) and Topic 3 (Periodicity), understanding electron affinity helps explain non-metal reactivity, halogen behavior, and periodic trends. It also helps clarify energy changes involved when atoms gain electrons to form negative ions.
What Is Electron Affinity?
Electron affinity is the energy change that occurs when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous negative ions.
General equation:
X(g) + e⁻ → X⁻(g)
IB definition focus:
- Atoms must be in the gaseous state
- One electron is added per atom
- Forms 1– ions
Electron affinity is usually expressed in kilojoules per mole (kJ/mol).
Is Electron Affinity Exothermic or Endothermic?
For most atoms:
- First electron affinity is exothermic
(Energy is released because the atom attracts the added electron.)
However:
- Second electron affinity is always endothermic, because adding an electron to an already negative ion involves repulsion.
Example:
O⁻ + e⁻ → O²⁻ (endothermic)
Atoms with strong attraction for an extra electron release more energy when that electron is gained.
Why Energy Is Released When Atoms Gain Electrons
When an electron approaches a neutral atom:
- The nucleus exerts an attractive force
- Energy is released when the electron becomes part of the atom
If the attraction is strong, electron affinity is large (very exothermic).
If the attraction is weak, electron affinity is small.
Electron affinity therefore reveals how much an atom “wants” an extra electron.
Factors Affecting Electron Affinity
Electron affinity depends on three main ideas:
1. Nuclear Charge
More protons → stronger attraction for incoming electrons → more exothermic electron affinity.
This is why halogens have large electron affinities.
2. Atomic Radius
Large atoms → electron farther from nucleus → weaker attraction → less exothermic electron affinity.
Smaller atoms attract electrons more strongly.
3. Electron–Electron Repulsion
If the added electron must enter an already crowded subshell, repulsion increases and electron affinity becomes less favorable.
Example:
Adding an electron to oxygen’s 2p subshell introduces repulsion.
Periodic Trends in Electron Affinity
Across a Period (left to right): becomes more exothermic
Why?
- Nuclear charge increases
- Atomic radius decreases
- Attraction for added electron increases
Result: Elements toward the right side (especially non-metals) gain electrons more readily.
Down a Group: becomes less exothermic
Why?
- Atomic radius increases
- Shielding increases
- Added electron is farther from the nucleus
Halogens still strongly attract electrons, but the effect weakens down the group.
Special Case: Halogens
Halogens (Group 17) have:
- High nuclear charge
- One electron away from noble-gas configuration
- Small atomic radii (top of group)
Thus, they have the most exothermic electron affinities in the periodic table.
Example:
Cl has a more exothermic electron affinity than F due to reduced electron–electron repulsion in the larger 3p orbital.
This is a classic IB exception.
First vs Second Electron Affinity
First Electron Affinity
Usually exothermic.
X(g) + e⁻ → X⁻(g)
Second Electron Affinity
Always endothermic because you are adding an electron to a negative ion.
X⁻(g) + e⁻ → X²⁻(g)
Example:
O(g) + e⁻ → O⁻(g) (exothermic)
O⁻(g) + e⁻ → O²⁻(g) (endothermic)
The strong repulsion explains why second electron affinities are positive.
Why Electron Affinity Matters in IB Chemistry
Electron affinity helps explain:
- Why halogens are so reactive
- Why noble gases rarely form ions
- Why metals tend to lose electrons rather than gain them
- Trends in non-metal reactivity
- Formation of anions in ionic bonding
It also reinforces atomic structure concepts like nuclear charge and shielding.
Common IB Misunderstandings
“Electron affinity is the same as electronegativity.”
No—electronegativity is attraction in a bond, while electron affinity is about adding an electron to a gaseous atom.
“Electron affinity always releases energy.”
First electron affinity usually does, second electron affinity does not.
“Smaller atoms always have higher electron affinity.”
Usually—but oxygen is an exception because of repulsion in the 2p subshell.
“All non-metals have high electron affinity.”
Only some do—sulfur and phosphorus have moderate values.
FAQs
Why do noble gases have almost no electron affinity?
Their outer shells are full, so adding an electron is energetically unfavorable.
Why is chlorine’s electron affinity more exothermic than fluorine’s?
Fluorine is very small, causing strong repulsion in the crowded 2p orbital.
Which elements have the highest electron affinities?
Halogens, especially chlorine and bromine.
Conclusion
Electron affinity is the energy change that occurs when a gaseous atom gains an electron. It depends on nuclear charge, atomic radius, and electron repulsion. Across a period, electron affinity becomes more exothermic; down a group, it becomes less exothermic. Understanding electron affinity helps IB Chemistry students explain reactivity, periodic trends, and the formation of negative ions.
