Anand Classes explains the important concept of electron gain enthalpy of halogens in a simple yet detailed way for JEE, NEET, and CBSE students. The trend of electron gain enthalpy in halogens (F, Cl, Br, I, At) is an essential topic of the periodic table, often asked in competitive exams. While generally electron gain enthalpy becomes less negative down the group, fluorine shows an anomaly where its value is less negative than chlorine. Understanding this exception, along with the overall trend, helps students strengthen their grasp on periodic properties and improve exam performance.
Why Electron Gain Enthalpy of Fluorine is Less Negative than Chlorine ?
General Trend of Electron Gain Enthalpy in a Group
- Electron gain enthalpy is the amount of energy released when an electron is added to a neutral isolated atom in the gaseous state.
- Across a group from top to bottom, the atomic size increases and the nuclear attraction for an additional electron decreases.
- As a result, the electron gain enthalpy becomes less negative as we move down a group.
- Example: Expected trend → F > Cl > Br > I > At (in terms of more negative values).
But Actual Trend : Cl > F > Br > I > At
- Contrary to expectation, fluorine does not have the most negative electron gain enthalpy.
- Instead, chlorine has the most negative electron gain enthalpy among the halogens.
Reason :
However, it is observed that F-atom has unexpectedly less negative electron gain enthalpy than Cl-atom. The anomaly arises because of the very small atomic size of fluorine.
(a) Small Size of Fluorine
- The fluorine atom has only two shells (n = 2), with the outermost electrons in the 2p subshell.
- This 2p subshell is very compact and can accommodate only six electrons.
(b) Strong Inter-electronic Repulsions
- When a new electron enters this compact 2p orbital, it faces strong repulsion from the already present electrons.
- These repulsions destabilize the incoming electron, reducing the amount of energy released.
(c) Effective Nuclear Attraction Reduced
- Although fluorine has a very high electronegativity and a strong nuclear charge, the repulsions in the small 2p orbital outweigh the attraction.
- Hence, the incoming electron does not feel as much attraction as expected.
Why Chlorine Has More Negative Electron Gain Enthalpy ?
- Chlorine is larger in size than fluorine and its outermost electron enters the 3p orbital.
- The 3p orbital is more diffused (larger volume), so the added electron experiences less repulsion from the existing electrons.
- Therefore, the nucleus can attract the incoming electron more effectively, leading to a greater release of energy.
- Hence, chlorine has the most negative electron gain enthalpy among the halogens.
Trend of Electron Gain Enthalpy in Halogens (Group 17)
General Character of Halogens
- Halogens (F, Cl, Br, I, At) are highly electronegative elements.
- They have a strong tendency to gain one electron to complete their outermost p-orbital (ns² np⁵ → ns² np⁶ configuration, i.e., noble gas).
- Hence, halogens possess large negative electron gain enthalpy values.
Expected Trend Down the Group
- As we go down the group (F → Cl → Br → I → At):
- Atomic size increases.
- Nuclear attraction for the incoming electron decreases.
- Electron enters into orbitals of higher principal quantum number (2p → 3p → 4p → 5p → 6p), which are larger and farther from the nucleus.
- Thus, electron gain enthalpy should become less negative.
So the expected order would be : F > Cl > Br > I > At(in negativity of EGE)
The Anomaly of Fluorine vs. Chlorine
- Fluorine is an exception: Its electron gain enthalpy is less negative than chlorine.
- Reason:
- Extremely small atomic size of fluorine.
- Incoming electron must enter the very compact 2p subshell.
- This causes strong electron–electron repulsions.
- As a result, the energy released is smaller.
Thus the actual order is: Cl > F > Br > I > At
Approximate Values of Electron Gain Enthalpy (in kJ/mol)
| Element | Electron Gain Enthalpy (ΔegH, kJ/mol) | Remark |
|---|---|---|
| Fluorine (F) | –328 | Less negative due to strong repulsions in small 2p orbital |
| Chlorine (Cl) | –349 | Most negative among all halogens |
| Bromine (Br) | –325 | Less negative than Cl (size effect) |
| Iodine (I) | –295 | Still less negative (large size, weak attraction) |
| Astatine (At) | ≈ –270 | Weakest attraction (very large size, least electronegative) |
Final Order of Electron Gain Enthalpy
Cl > F > Br > I > At
✅ Final Key Point:
Although fluorine is the most electronegative element, it does not have the most negative electron gain enthalpy. Instead, chlorine does, due to the balance between nuclear attraction and inter-electronic repulsions.
📘 Important Questions & Answers on Electron Gain Enthalpy of Halogens
Q1. Define electron gain enthalpy. How does it vary across a period and down a group?
Answer:
- Definition: Electron gain enthalpy (ΔegH) is the energy change when an isolated gaseous atom gains an electron to form a gaseous anion.
X(g) + e− → X(g)− + ΔH
- Across a period: Becomes more negative from left to right due to increasing nuclear charge and decreasing atomic size.
- Down a group: Becomes less negative because atomic size increases and nuclear attraction decreases.
Q2. Why is the electron gain enthalpy of chlorine more negative than that of fluorine?
Answer:
- Fluorine is very small, and the incoming electron enters the compact 2p orbital.
- Strong electron–electron repulsions reduce the attraction from the nucleus.
- In chlorine, the incoming electron enters the larger 3p orbital, where repulsion is less, and attraction is stronger.
✅ Therefore, chlorine has more negative electron gain enthalpy than fluorine.
Q3. Arrange the halogens (F, Cl, Br, I) in the order of electron gain enthalpy and explain.
Answer:
Order: Cl > F > Br > I > At
- Chlorine has the most negative ΔegH (–349 kJ/mol).
- Fluorine is slightly less negative due to repulsion in its 2p orbital.
- Bromine and iodine show less negative values as size increases and attraction decreases.
Q4. Why do second-period elements (O, F) have less negative electron gain enthalpies than third-period elements (S, Cl)?
Answer:
- Second-period elements have very small atomic size.
- The extra electron enters a compact orbital → strong inter-electronic repulsions.
- This reduces energy release.
✅ Therefore, O and F have less negative ΔegH compared to S and Cl.
Q5. Which halogen has the highest tendency to accept an electron and why?
Answer:
- Chlorine has the highest tendency to accept an electron.
- Reason: It has the most negative electron gain enthalpy (–349 kJ/mol).
- Due to optimum size, incoming electron experiences strong attraction without excessive repulsion.
Q6. Explain why noble gases have positive electron gain enthalpy values.
Answer:
- Noble gases already have stable ns² np⁶ configuration.
- Addition of an extra electron requires energy (unstable configuration).
- Hence, ΔegH is positive for noble gases.
Q7. Assertion-Reason Question (Exam Pattern)
Assertion (A): The electron gain enthalpy of chlorine is more negative than that of fluorine.
Reason (R): In fluorine, strong electron–electron repulsions are present in the compact 2p orbital.
✅ Answer: Both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.
📚 Buy Study Material & Join Our Coaching
For premium study materials specially designed for JEE, NEET, NDA, and CBSE/ICSE Classes, visit our official study material portal:
👉 https://publishers.anandclasses.co.in/
To enroll in our offline or online coaching programs, visit our coaching center website:
👉 https://anandclasses.co.in/
📞 Call us directly at: +91-94631-38669
💬 WhatsApp Us Instantly
Need quick assistance or want to inquire about classes and materials?
📲 Click below to chat instantly on WhatsApp:
👉 Chat on WhatsApp
🎥 Watch Video Lectures
Get access to high-quality video lessons, concept explainers, and revision tips by subscribing to our official YouTube channel:
👉 Neeraj Anand Classes – YouTube Channel