Electron Affinity (EA) is defined as the amount of energy released when a neutral, isolated gaseous atom accepts an electron to form a negatively charged ion (anion). It measures the electron-attracting tendency of an isolated atom.
Thermodynamic Sign Conventions
- First Electron Affinity (EA1): Generally exothermic (energy is released), meaning EA1 carries a negative enthalpy value (Δ Heg < 0). This is because the incoming electron is attracted by the positive nucleus of the neutral atom.
- Second Electron Affinity (EA2): Always endothermic (energy is absorbed), meaning EA2 carries a positive enthalpy value (Δ Heg > 0). This occurs because energy must be supplied to overcome the strong electrostatic repulsion between the existing uninegative anion (X^-) and the incoming electron.
Key Factors Influencing Electron Affinity
Nuclear Charge
Electron affinity is directly proportional to the nuclear charge. As the nuclear charge increases, the effective attractive force exerted by the nucleus on the incoming electron increases, resulting in a higher release of energy.
Atomic Size
Electron affinity is inversely proportional to atomic size. In smaller atoms, the incoming electron is bound closer to the nucleus, experiencing a stronger gravitational pulling force, which leads to higher electron affinity values.
Electronic Configuration
Atoms possessing stable electronic configurations—specifically half-filled or fully-filled valence subshells—exhibit extremely low or zero electron affinity. They resist adding an electron because doing so disrupts their inherent structural stability.
Periodic Trends in Electron Affinity
Variations Across a Period (Left to Right)
- Trend: Electron affinity generally increases (becomes more negative) across a period from left to right.
- Reason: Atomic size decreases and nuclear charge increases sequentially, which enhances the nucleus’s ability to attract external electrons.
Variations Down a Group (Top to Bottom)
- Trend: Electron affinity generally decreases (becomes less negative) down a group.
- Reason: The addition of new electronic shells increases atomic radius significantly. The shielding effect reduces the nucleus’s hold on the outermost shell, lowering the energy released upon electron addition.
Notable Trend Anomalies
| Anomalous Pair | Expected Trend | Actual Observation | Scientific Cause |
| Fluorine vs. Chlorine | F > Cl | Cl > F | Fluorine has an exceptionally small atomic size ($2porbital). The incoming electron faces severe inter-electronic repulsion from the tightly packed cloud, reducing the net energy released. Chlorine (%%MONEYBLOCK1%%p orbital) has more space to accommodate the electron easily. |
| Oxygen vs. Sulfur | O > S | S > O | Like Fluorine, Oxygen’s small $2pshell creates high electron density repulsion, making its electron affinity lower than that of Sulfur (%%MONEYBLOCK3%%p). |
| Group 2 Elements (Be, Mg) | High affinity | Near Zero / Positive | They have a completely filled stable ns2 subshell. The incoming electron must enter a higher energy p-orbital, which is energetically unfavorable. |
| Group 15 Elements (N, P) | Higher than Group 14 | N is lower than C | Nitrogen has a stable, exactly half-filled electronic configuration ($2p^3). Adding an electron creates inter-electronic pairing repulsion, disrupting stability.</td> </tr> <tr> <td><b>Group 18 Elements (Noble Gases)</b></td> <td>High affinity</td> <td>Zero or Positive</td> <td>They feature completely filled octets (ns^2 np^6). The incoming electron must occupy a new principal energy shell, requiring external energy input.</td> </tr> </tbody> </table> <h4>Comparison Matrix: Electron Affinity vs. Electronegativity</h4> <h5>Structural Differences</h5> <p> Electron affinity is a quantitative, measurable thermodynamic property of an isolated gaseous atom, expressed in units like\text{kJ/mol}or\text{eV/atom}. Electronegativity is a qualitative, dimensionless property representing the relative tendency of an bonded atom to attract shared electron pairs in a covalent chemical bond. </p> <h5>Environmental Conditioning</h5> <p> Electron affinity operates independently on single atoms in the gaseous phase. Electronegativity is an environment-dependent attribute evaluated only when atoms are chemically combined within a molecule. </p> <h4>UPSC Prelims Fact File and Trivia</h4> <h5>Highest and Lowest Elements</h5> <ul> <li> <b>Highest Electron Affinity:</b> Chlorine (\text{Cl}) holds the highest electron affinity among all elements in the periodic table (-349 \text{ kJ/mol}), surpassing Fluorine. </li> <li> <b>Lowest/Zero Electron Affinity:</b> Noble gases like Neon (\text{Ne}) and Argon (\text{Ar}$) exhibit virtually zero or positive electron affinities due to their closed-shell electronic configuration.Halogens as a GroupGroup 17 elements (Halogens) have the highest electron affinities in their respective periods because they are just one electron short of attaining a stable noble gas configuration. Periodic Trend Order for HalogensThe precise decreasing order of electron affinity for halogens is: Cl > F > Br > I Last Modified: May 25, 2026
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