Anand Classes brings you a detailed explanation of the important topic Factors Affecting Electron Gain Enthalpy for JEE, NEET, and CBSE Class 11 Chemistry. Electron gain enthalpy refers to the energy change when an isolated gaseous atom gains an extra electron to form a negative ion. Understanding how nuclear charge, atomic size, electronic configuration, and shielding effect influence electron gain enthalpy is crucial for mastering periodic trends and solving exam-based questions. This article provides clear theory, examples, FAQs, and comparisons to help students build strong conceptual clarity and score high in competitive as well as board examinations.

Factors on which Electron Gain Enthalpy Depends

Electron gain enthalpy is the amount of energy released (or absorbed) when an atom in the gaseous state accepts an extra electron to form a negative ion. Its value is influenced by several factors:

(i) Nuclear Charge

• The greater the nuclear charge, the stronger will be the attraction between the positively charged nucleus and the incoming electron.
• A stronger attraction means more energy is released when the electron is added, making the electron gain enthalpy more negative.
• Example:
  • Fluorine (Z = 9) has a high nuclear charge for its small size, hence it has a highly negative electron gain enthalpy.
  • Sodium (Z = 11) has lower effective nuclear charge due to more inner electrons shielding the nucleus, so its electron gain enthalpy is less negative.

(ii) Size of the Atom

• As the atomic radius increases, the distance between the nucleus and the incoming electron also increases.
• A larger distance reduces the electrostatic attraction, so less energy is released when the electron is gained.
• Hence, electron gain enthalpy becomes less negative with increasing size of the atom.
• Example:
  • Chlorine (Cl) has a smaller atomic size compared to iodine (I). Therefore, chlorine has a more negative electron gain enthalpy than iodine.

(iii) Electronic Configuration

• Atoms with stable electronic configurations (such as noble gases, half-filled, or fully filled subshells) show very little tendency to gain electrons.
• In such cases, either:
  • Energy has to be supplied to force an electron into the stable configuration (making ΔegH positive), or
  • The energy released is very small (making ΔegH only slightly negative).
• Examples:
  • Noble gases (He, Ne, Ar) have completely filled shells, so they resist gaining electrons → ΔegH is positive.
  • Nitrogen (1s² 2s² 2p³) has a half-filled stable 2p subshell; hence its electron gain enthalpy is less negative than oxygen, even though oxygen has a smaller size.

(iv) Shielding Effect (Screening Effect)

• The electrons present in the inner shells repel the incoming electron and shield it from the full attraction of the nucleus.
• Higher the number of inner-shell electrons, greater will be the shielding, and thus weaker will be the effective nuclear attraction felt by the incoming electron.
• As a result, the electron gain enthalpy becomes less negative.
• Example:
  • In halogens, fluorine and chlorine have similar nuclear charges, but in chlorine, the added electron experiences slightly less repulsion (less crowding of electrons) compared to fluorine. That’s why chlorine actually has a more negative electron gain enthalpy than fluorine.

❓ FAQs on Electron Gain Enthalpy (Examination Asked Conceptual Questions & Answers)

Q1. Why is electron gain enthalpy of chlorine more negative than that of fluorine, even though fluorine is smaller in size?

Answer:

• Normally, a smaller size means stronger attraction between the nucleus and the incoming electron, so ΔegH should be more negative.
• But in fluorine, the atomic size is very small, and the incoming electron has to enter the compact 2p orbital, where strong electron–electron repulsions are present due to crowding of electrons.
• In chlorine, the incoming electron enters the larger 3p orbital, where repulsions are less significant.
• Therefore, chlorine releases more energy on accepting an electron → ΔegH of chlorine is more negative (–349 kJ mol⁻¹) compared to fluorine (–328 kJ mol⁻¹).

Q2. Why do noble gases have positive electron gain enthalpy?

Answer:

• Noble gases (He, Ne, Ar, etc.) have completely filled electronic configurations (ns² np⁶).
• This configuration is very stable, and adding an electron would require placing it in the next higher energy orbital (new shell).
• Instead of releasing energy, the atom would need extra energy input to force the electron in.
• Thus, electron gain enthalpy of noble gases is positive.

Q3. Why is the electron gain enthalpy of nitrogen less negative than that of oxygen?

Answer:

• Nitrogen: 1s² 2s² 2p³ → has a half-filled stable p-orbital configuration.
• Adding an electron would disturb this stability, so nitrogen has very low tendency to accept an electron → ΔegH is less negative.
• Oxygen: 1s² 2s² 2p⁴ → does not have a half-filled stable configuration, so it can accept an electron more easily to form O⁻ with 2p⁵.
• Hence, oxygen has a more negative electron gain enthalpy than nitrogen.

Q4. Why does electron gain enthalpy become less negative down a group?

Answer:

• As we move down a group, the atomic size increases and shielding effect increases.
• The nucleus is farther from the incoming electron, and inner electrons reduce the effective nuclear attraction.
• As a result, less energy is released when an extra electron is added → ΔegH becomes less negative down the group.
• Example: ΔegH(Cl) is more negative than ΔegH(Br) or ΔegH(I).

Q5. Why does electron gain enthalpy become more negative across a period (left to right)?

Answer:

• Across a period:
  • Nuclear charge increases.
  • Atomic size decreases.
• Both factors increase the effective attraction between nucleus and incoming electron.
• Thus, atoms on the right side of the periodic table (non-metals like halogens) release more energy when gaining an electron.
• Example: ΔegH of halogens is highly negative, while for alkali metals (Na, K) it is low.

Q6. Which element has the most negative electron gain enthalpy?

Answer:

• Among all elements, chlorine (Cl) has the most negative electron gain enthalpy (≈ –349 kJ mol⁻¹).
• Reason:
  • High nuclear charge.
  • Optimal atomic size (not too small like F, not too large like Br or I).
  • Low electron–electron repulsions compared to fluorine.
• Hence, chlorine is the most favorable element for gaining an electron.

Q7. Why is electron gain enthalpy of oxygen less negative than that of sulfur, even though oxygen is smaller in size?

Answer:

• Oxygen is very small, so when an extra electron is added, it experiences strong electron–electron repulsions in the compact 2p orbital.
• In sulfur, the incoming electron enters the larger 3p orbital, where repulsions are less.
• Thus, sulfur has a more negative electron gain enthalpy than oxygen, even though oxygen is smaller.

📊 Comparison Table: Factors Affecting Electron Gain Enthalpy

✅ Quick Revision Points

• High nuclear charge → Δ_egH more negative.
• Large atomic size → Δ_egH less negative.
• Stable configuration → Δ_egH less negative or positive.
• Strong shielding effect → Δ_egH less negative.

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