Overview
This lesson explains electron affinity, the energy change when an atom gains an electron, and describes trends and exceptions in the periodic table.
Electron Affinity: Definition and Process
- Electron affinity is the energy change when a neutral atom gains an electron.
- The process usually releases energy, recorded as a negative value.
- Electron affinity measurements are made for gaseous atoms and are difficult to measure precisely.
Trends in Electron Affinity
- Electron affinity becomes more negative (greater energy released) from left to right across a period.
- Electron affinity becomes less negative (less energy released) from top to bottom within a group.
- Halogens (Group 17) have the largest (most negative) electron affinities, favoring anion formation.
- Noble gases have little or zero electron affinity because their outer shells are full and stable.
- Trends in electron affinities are less regular than for ionization energy, with several exceptions.
Factors Affecting Electron Affinity
- As atomic size increases down a group, the added electron is farther from the nucleus, so less energy is released.
- Halogens have a nearly full valence shell, so gaining an electron provides extra stability, releasing more energy.
- Measurement difficulties and electron configuration anomalies cause exceptions in periodic trends.
Frequently Asked Questions & Examples
- Addition of an electron to a neutral atom releases energy.
- Chlorine has the most negative electron affinity among elements.
- Noble gases have no electronegativity due to their full valence shells.
- Larger atoms down a group have smaller electron affinities because the added electron is farther from the nucleus.
- Bromine has a higher boiling point than chlorine due to stronger London dispersion forces.
Key Terms & Definitions
- Electron affinity — the energy change when an atom gains an electron, usually negative as energy is released.
- Anion — a negatively charged ion formed by gaining one or more electrons.
- Halogen — elements in Group 17, which readily gain electrons.
- Noble gas — elements in Group 18 with full valence shells and low reactivity.
- Ionization energy — energy needed to remove an electron from an atom.
Action Items / Next Steps
- Review trends of electron affinity across periods and groups on the periodic table.
- Practice identifying which elements have highest or lowest electron affinities.
- Answer review questions about electron affinity trends and exceptions.