Periodic Trend: Electron Affinity (Simplified)

Periodic Trend: Electron Affinity

Definition of Electron Affinity

Electron Affinity (EA) is the energy released when an electron is added to a gaseous atom or ion. It is a measure of an atom's tendency to accept an electron.

• Exothermic Reaction: A negative value of EA means the element will most readily accept an electron.

• Reason: Largely related to stable electron configurations (e.g., noble gases have low EA).

Equation:

$\text{X} (g) + e^- \rightarrow \text{X}^- (g) + \text{Energy}$

Periodic Trend of Electron Affinity

Electron affinity generally increases (becomes more negative) across a period (left to right) and decreases (becomes less negative) down a group in the periodic table.

• Across a Period: EA increases (atoms more readily accept electrons).

• Down a Group: EA decreases (atoms less readily accept electrons).

Reason: Atoms with nearly filled valence shells (e.g., halogens) have high electron affinities, while noble gases and group 2 elements have low or positive EA values due to stable configurations.

Periodic Table Trend Illustration

The periodic table highlights the trend, with electron affinity increasing toward the upper right (excluding noble gases).

Group	1A	2A	3A	4A	5A	6A	7A	8A
EA Trend	Low	Low	Moderate	Moderate	High	Very High	Highest	Very Low/Positive
Example Elements	Na	Mg	Al	Si	P	O	F	Ne

Additional info: Noble gases (Group 8A) have nearly zero or positive electron affinities due to their full valence shells.

Examples and Applications

• Example: Which of the following halogens will release the most energy with the addition of an electron? Answer: F (Fluorine)

• Practice: Determine which atom in the following set has the largest electron affinity: N, O, C, B. Answer: O (Oxygen)

Practice Problems

• Rank the following elements in order of increasing electron affinity: Ca, Hg, F, S. Order: Hg < F < S < Ca

• Which one of the following atoms has the least tendency to gain another electron? Answer: Oxygen

• Arrange the following elements from greatest to least tendency to accept an electron: F, Li, C, O, Be. Answer: Chlorine

Summary Table: Electron Affinity Trends

Element	Electron Affinity (EA)	Tendency to Accept Electron
F (Fluorine)	Very High (most negative)	Greatest
O (Oxygen)	High	High
C (Carbon)	Moderate	Moderate
Be (Beryllium)	Low (positive or near zero)	Least

Additional info: Electron affinity values can be used to predict chemical reactivity, especially for nonmetals.