BackElectron Affinity: Concepts, Trends, and Practice Problems
Study Guide - Smart Notes
Tailored notes based on your materials, expanded with key definitions, examples, and context.
Electron Affinity (EA)
Definition and Explanation
Electron Affinity (EA) is the energy released when an electron is added to a gaseous atom or ion. It is typically measured in kilojoules per mole (kJ/mol). The process can be represented as:
High electron affinity means the element readily accepts an electron, releasing more energy.
Low electron affinity means the element does not easily accept an electron.
Exceptions and Stability
Exceptions: Elements with stable electron configurations (such as noble gases) have low or even positive electron affinities, meaning they do not readily accept electrons.
Reason: Uniquely stable electron arrangements resist change, so adding an electron is energetically unfavorable.
Periodic Trends in Electron Affinity
Electron affinity varies across the periodic table:
Across a Period (left to right): Electron affinity generally increases as atoms more readily accept electrons.
Down a Group (top to bottom): Electron affinity generally decreases as atomic size increases and the added electron is farther from the nucleus.
Summary Table:
Direction | Trend in Electron Affinity |
|---|---|
Left to Right (Period) | Increases |
Top to Bottom (Group) | Decreases |
Additional info: Halogens (Group 7A) have the highest electron affinities, while noble gases (Group 8A) have very low or positive electron affinities.
Electron Affinity on the Periodic Table
The periodic table highlights the trend:
Elements on the right (especially halogens) have high electron affinity.
Elements on the left and noble gases have low electron affinity.
Examples and Applications
Example: Among halogens, chlorine (Cl) releases the most energy when accepting an electron.
Practice: Oxygen (O) has the largest electron affinity among N, O, C, B, Ne.
Practice Problems
Rank the following elements in order of increasing electron affinity: Cs, Hg, F, S Answer: Cs < Hg < S < F
Which atom has the least tendency to gain another electron? Answer: Neon (Ne)
Arrange the following elements from greatest to least tendency to accept an electron: F, Li, C, O, Be Answer: F > O > C > Li > Be
Key Terms
Electron Affinity (EA): Energy change when an electron is added to a neutral atom in the gas phase.
Halogens: Elements in Group 7A, known for high electron affinity.
Noble Gases: Elements in Group 8A, known for low or positive electron affinity.
Summary Table: Electron Affinity of Selected Elements
Element | Electron Affinity (kJ/mol) | Tendency to Accept Electron |
|---|---|---|
Fluorine (F) | ~328 | Very High |
Oxygen (O) | ~141 | High |
Neon (Ne) | ~0 (positive) | Very Low |
Cesium (Cs) | ~-45 | Low |
Additional info: Electron affinity values can be positive (energy absorbed) for some elements, indicating resistance to electron addition.
