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Periodic Trends: Electron Affinity

Watch a gaseous atom capture an electron and release energy

Neutral atom

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Choose a gaseous atom

Chlorine

—EA eV

—kJ/mol released

3ptarget subshell

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Electron-capture experiment

Incoming electron kinetic energy0.20 eV

Ready. The atom and electron begin far apart in the gas phase.

Convention here: a larger positive value means more energy is released by first-electron attachment.

Compare two atoms

Energy history

Purple: released energy • Teal: electron distance

Captured object

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3D camera

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3D orbital probability density

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Cloud density220

Live subshell energy diagram

First versus second attachment

First attachment begins with a neutral atom.

Interactive periodic-table heat map

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Correlation plotter

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Exception investigator

Select an exception to investigate orbital crowding and subshell stability.

Classroom investigation

What is electron affinity?

Electron affinity describes the energy change when an isolated gaseous atom accepts an electron and becomes a gaseous negative ion.

Studio convention: positive numbers show the magnitude of energy released. In thermodynamic enthalpy notation, the same favorable process may be written with a negative sign.

What controls it?

Nuclear attraction

A stronger effective positive charge can attract the incoming electron more strongly.

↔

Distance and shielding

Larger distance and inner-electron shielding usually weaken attraction.

⇅

Repulsion and configuration

Crowded compact orbitals, filled subshells, and half-filled subshells create important exceptions.

Periodic pattern and exceptions

Across a period, electron affinity generally becomes more favorable, but the pattern is irregular.
Halogens are strongly favorable because one electron completes the valence p subshell.
Chlorine releases slightly more energy than fluorine because fluorine’s compact 2p region has stronger electron-electron repulsion.
Group 2 and Group 15 often dip because filled s and half-filled p arrangements are relatively stable.
Noble gases resist attachment because the extra electron must begin a higher shell.
Second attachment is unfavorable because an electron is added to an ion that is already negative.

Learning objectives

Define first electron affinity for gaseous atoms.
Predict broad trends using attraction, distance, shielding, and repulsion.
Explain Cl versus F and the Group 2, Group 15, and noble-gas exceptions.
Connect orbital occupancy to energy release.
Interpret heat maps and comparison plots without treating the trend as perfectly smooth.

Suggested experiments

Compare F, Cl, Br, and I, then explain why Cl is the maximum.
Compare Be with B and N with O.
Launch the electron at a halogen and at a noble gas.
Switch to second attachment and observe electrostatic repulsion.

Questions for exploration

Why is electron affinity more irregular than ionization energy?
What does a missing experimental value mean?
How does orbital size affect repulsion?

Element property data embedded from the PubChem periodic table. Missing values are shown as unavailable rather than estimated.

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