Variation of Electron Gain Enthalpy Down a Group and Along a Period | Conceptual Important Question Answers

Anand Classes provides the best study material for JEE, NEET, and CBSE students. In this article, we explain one of the most important periodic properties – Electron Gain Enthalpy. It refers to the energy change when an atom gains an extra electron to form a negative ion. Understanding the periodic trends of electron gain enthalpy (variation across a period and down a group) is crucial for competitive exams like JEE Main, JEE Advanced, NEET, and Board examinations, as it helps in predicting the reactivity and chemical behavior of elements.

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Periodic Trends in Electron Gain Enthalpy

Electron gain enthalpy refers to the energy change that occurs when an electron is added to a neutral gaseous atom. Its trend in the periodic table can be summarized as follows:

• Across a Period (Left to Right):
  Electron gain enthalpy generally becomes more negative.
• Down a Group (Top to Bottom):
  Electron gain enthalpy generally becomes less negative.

Let us understand these variations in detail:

Variation of Electron Gain Enthalpy Down a Group

• As we move down a group, both the atomic size and nuclear charge increase.
• However, the increase in atomic size outweighs the effect of increased nuclear charge.
• As a result, the added electron experiences less attraction towards the nucleus in larger atoms.
• Therefore, the electron gain enthalpy becomes less negative on moving down a group.

👉 This trend is clearly seen in the halogen group, where the electron gain enthalpy decreases from chlorine → bromine → iodine.

Variation of Electron Gain Enthalpy Along a Period

• As we move across a period (left to right), the atomic size decreases while the nuclear charge increases.
• Both these factors enhance the attraction between the nucleus and the incoming electron.
• Consequently, the electron gain enthalpy becomes more negative across a period.

⚠️ However, some irregularities occur in this trend. These are mainly due to the presence of stable electronic configurations (e.g., completely filled or half-filled orbitals) in certain elements, which resist the addition of extra electrons.

Perfect 👍 You’ve shared Table 18: Electron Gain Enthalpies of Some Elements. I’ll rewrite it neatly in a well-structured tabular format, correcting small formatting issues and making it clear for your notes:

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📊 Electron Gain Enthalpies of Some Elements (kJ mol⁻¹)

Element	H	He	Li	Be	B	C	N	O	F	Ne
ΔegH	–73	+48	–60	≈0	–83	–127	≈0	–141	–328	+116

Element	Na	Mg	Al	Si	P	S	Cl	Ar
ΔegH	–53	≈0	–50	–119	–74	–200	–349	+97

Element	K	Ca	Ga	Ge	As	Se	Br	Kr
ΔegH	–48	–2	–36	–116	–77	–195	–325	+96

Element	Rb	Sr	Te	I	Xe
ΔegH	–47	–5	–190	–295	+77

Element	Cs	Po	At	Rn
ΔegH	–46	–174	–270	+68

✅ Notes on the table:

• Negative values indicate energy released (more negative → stronger tendency to gain electron).
• Positive values indicate energy absorbed (unfavourable to gain an electron).
• The most negative value is for Chlorine (–349 kJ mol⁻¹).
• Noble gases (He, Ne, Ar, Kr, Xe, Rn) have positive values due to stable configurations.
• Irregularities (like N, Be, Mg) occur because of stable half-filled or fully filled orbitals.

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📘 Electron Gain Enthalpy – FAQs & Conceptual Questions

Q1. What is electron gain enthalpy?

👉 Electron gain enthalpy is the energy change (usually expressed in kJ/mol) that occurs when an electron is added to a neutral gaseous atom to form a negative ion.

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Q2. Why is electron gain enthalpy usually negative?

👉 When an atom gains an electron, energy is usually released because of the electrostatic attraction between the nucleus and the incoming electron. Hence, electron gain enthalpy is negative for most elements.

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Q3. Why does electron gain enthalpy become less negative down a group?

👉 Down a group:

• Atomic size increases.
• Effective nuclear attraction on the added electron decreases.
  Thus, less energy is released, making electron gain enthalpy less negative.

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Q4. Why does electron gain enthalpy become more negative across a period?

👉 Across a period:

• Atomic size decreases.
• Nuclear charge increases.
  So, the added electron experiences greater attraction, releasing more energy. Hence, electron gain enthalpy becomes more negative.

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Q5. Why is electron gain enthalpy of chlorine more negative than that of fluorine, although fluorine is smaller in size?

👉 In fluorine, the very small size of the atom leads to high electron-electron repulsion in its compact 2p orbital. This reduces the tendency to accept an extra electron.
Thus, Cl (–349 kJ/mol) has a more negative electron gain enthalpy than F (–328 kJ/mol).

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

👉 Chlorine (Cl) has the most negative electron gain enthalpy, not fluorine (due to electron repulsion as explained above).

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Q7. Why do noble gases have positive electron gain enthalpies?

👉 Noble gases have completely filled stable configurations (ns² np⁶). Adding an electron would require moving it to a higher energy orbital, which is energetically unfavourable. Hence, their electron gain enthalpy values are positive.

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Q8. Why do elements with half-filled or fully filled subshells show irregularities in electron gain enthalpy trends?

👉 Half-filled (e.g., N: 2p³) and fully filled subshells (e.g., Be: 2s², Mg: 3s²) are stable configurations. Adding an extra electron disturbs this stability. Hence, such elements show less negative or even positive electron gain enthalpy values.

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🎯 Conceptual Examination Questions & Answers

Q1. Arrange the following in increasing order of electron gain enthalpy: O, F, S, Cl

👉 Answer:
Trend: More negative across a period, less negative down a group.

• O (–141 kJ/mol), S (–200 kJ/mol), F (–328 kJ/mol), Cl (–349 kJ/mol).

So, order is: O < S < F < Cl.

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Q2. Why is the electron gain enthalpy of nitrogen less negative than that of oxygen?

👉 Answer:

• Nitrogen has a stable half-filled configuration (2p³).
• Adding an electron requires pairing in one orbital, causing repulsion.
• Oxygen (2p⁴) readily accepts an electron to complete a more stable half-filled orbital pair.
  Thus, nitrogen has less negative electron gain enthalpy than oxygen.

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Q3. Between Cl and Br, which has a more negative electron gain enthalpy and why?

👉 Answer:

• Chlorine has a smaller atomic size compared to bromine.
• Hence, the added electron in chlorine experiences stronger nuclear attraction.
• Therefore, electron gain enthalpy of Cl is more negative than that of Br.

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Q4. Explain why electron gain enthalpy of fluorine is less negative than chlorine, even though fluorine is more electronegative.

👉 Answer:

• Fluorine’s extremely small size causes high electron-electron repulsion in its compact 2p orbital.
• This reduces energy released during electron addition.
• Hence, chlorine has a more negative electron gain enthalpy than fluorine.

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Q5. Predict and explain the sign of electron gain enthalpy of noble gases.

👉 Answer:

• Noble gases have stable octet configurations.
• Adding an electron requires placing it in a new higher-energy orbital (ns² np⁶ (n+1)s¹), which is energetically unfavourable.
• Therefore, their electron gain enthalpy is positive.

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🌟 Do You Know? – Electron Gain Enthalpy

🔹 Most Negative Electron Gain Enthalpy → Chlorine (Cl), not Fluorine.
(Reason: Less electron-electron repulsion compared to Fluorine.)

🔹 Positive Electron Gain Enthalpy → Noble gases (He, Ne, Ar, etc.) and some alkaline earth metals (like Be, Mg) have positive values, since adding an electron is energetically unfavourable.

🔹 Oxygen vs Sulphur → Oxygen has less negative electron gain enthalpy than sulphur because of strong electron-electron repulsion in its smaller 2p orbital.

🔹 Second Electron Gain Enthalpy → Always positive.
Reason: Adding an electron to an already negative ion requires extra energy to overcome electron-electron repulsion.
Example: O⁻ + e⁻ → O²⁻ (absorbs energy).

🔹 Units → Electron gain enthalpy is usually expressed in kJ mol⁻¹.

🔹 Trend Exceptions → Elements with stable half-filled or fully filled subshells (like N, Be, Mg) show irregularities in electron gain enthalpy trends.

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