Anand Classes brings you a detailed explanation of why halogens have the highest negative electron gain enthalpies in the periodic table. Halogens (Group 17 elements) like fluorine, chlorine, bromine, iodine, and astatine possess the electronic configuration ns²np⁵, which makes them just one electron short of achieving the stable noble gas configuration. This unique property gives halogens a very strong tendency to accept an additional electron, releasing a large amount of energy in the process. As a result, their electron gain enthalpies are highly negative, a key concept often asked in JEE, NEET, and CBSE Class 11 Chemistry exams.
Halogens and Electron Gain Enthalpy
General Concept
- Electron Gain Enthalpy (ΔegH): It is the amount of energy released (or absorbed) when an isolated gaseous atom accepts an electron to form a gaseous anion.
- A more negative value means the atom has a stronger tendency to accept electrons.
Halogens Have the Highest Negative electron gain enthalpies
- Halogens belong to Group 17 (F, Cl, Br, I, At).
- Their general electronic configuration is: ns2np5
- This means they have 7 valence electrons, just one electron short of achieving the stable noble gas configuration (ns2np6).
- Hence, they have a very strong tendency to gain one electron.
- As a result, their electron gain enthalpies are highly negative.
Reason for Highly Negative Values
- High Effective Nuclear Charge (Zeff): Halogens have a strong pull on incoming electrons due to high nuclear charge.
- Small Atomic Size (especially Fluorine and Chlorine): The smaller the size, the stronger the attraction between the nucleus and the incoming electron.
- Stability Factor: Achieving octet configuration (noble gas state) makes them highly stable, so they readily gain one electron.
Thus, halogens release a large amount of energy on gaining an electron, making ΔegH values highly negative.
Exception: Fluorine vs Chlorine
- Although fluorine is the smallest atom among halogens, its electron gain enthalpy is slightly less negative than chlorine.
- Reason: In fluorine, the incoming electron enters the very small 2p orbital, where electron–electron repulsions are high.
- In chlorine, the electron enters the larger 3p orbital, where repulsions are less.
- Therefore: ΔegH Cl < ΔegH F (in terms of negativity: Cl has a more negative value than F).
Order of Electron Gain Enthalpy (most negative → least negative)
Cl > F > Br > I > At
✅ Final Note:
Halogens, due to their electronic configuration and strong drive to complete the octet, have the most negative electron gain enthalpies in the periodic table.
FAQs (Frequently Asked Questions)
Q1. Why do halogens have the most negative electron gain enthalpies in the periodic table?
Answer:
- Halogens (Group 17 elements) have the general electronic configuration ns²np⁵.
- They need only one electron to achieve a stable noble gas configuration.
- They have high effective nuclear charge and small atomic size (especially F and Cl).
- Hence, they show a strong tendency to accept an extra electron, releasing large energy.
- Thus, their electron gain enthalpies are highly negative.
Q2. Why is the electron gain enthalpy of chlorine more negative than fluorine?
Answer:
- Fluorine is smaller in size, so the incoming electron faces strong repulsion from the already crowded 2p orbitals.
- In chlorine, the added electron goes into the larger 3p orbital, where repulsions are less.
- Hence, chlorine releases more energy than fluorine when gaining an electron.
ΔegH Cl < ΔegH F(in terms of negativity, Cl is more negative)
Q3. What is the trend of electron gain enthalpy in halogens?
Answer:
Order of negativity: Cl > F > Br > I > At
Q4. Why do noble gases have positive electron gain enthalpies, while halogens have negative values?
Answer:
- Noble gases already have stable ns²np⁶ configuration, so they resist accepting electrons. Energy must be supplied → positive values.
- Halogens are just one electron short of octet, so they strongly attract an extra electron and release energy → negative values.
Q5. Which halogen has the least negative electron gain enthalpy? Why?
Answer:
- Astatine (At) has the least negative value.
- Reason: It is the largest halogen atom with lowest effective nuclear charge and poorest attraction for an additional electron.
Conceptual Questions with Detailed Answers
Q1. Explain why halogens act as strong oxidising agents with the help of electron gain enthalpy.
Answer:
- Halogens readily accept an electron (highly negative electron gain enthalpy).
- By gaining an electron, they get reduced, but in this process, they cause oxidation of other species.
- Thus, halogens are powerful oxidising agents.
Example:
Cl2 + 2e− → 2Cl
Chlorine oxidises metals and non-metals easily because of its highly negative ΔegH.
Q2. Why does the electron gain enthalpy become less negative down the halogen group?
Answer:
- As we move down the group (Cl → Br → I → At), the atomic size increases and the nuclear attraction decreases.
- The incoming electron feels less pull from the nucleus.
- Hence, energy released decreases → electron gain enthalpy becomes less negative.
Q3. Compare and explain the electron gain enthalpy of halogens with oxygen and sulphur (Group 16).
Answer:
- Halogens (ns²np⁵) need only 1 electron for octet, while oxygen and sulphur (ns²np⁴) need 2 electrons.
- Therefore, halogens have greater tendency to accept electrons.
- Thus, halogens have more negative electron gain enthalpies than Group 16 elements.
Q4. If chlorine has more negative electron gain enthalpy than fluorine, why is fluorine still considered the strongest oxidising agent?
Answer:
- Oxidising power depends not only on electron gain enthalpy but also on:
- Bond dissociation enthalpy of X–X bond
- Hydration enthalpy of X⁻ ion
- Fluorine has a very low bond dissociation enthalpy (F–F bond is weak) and very high hydration enthalpy of F⁻ ion.
- These factors compensate for its less negative ΔegH, making fluorine the strongest oxidising agent.
Quick Revision Points
- Halogens have most negative electron gain enthalpies due to ns²np⁵ configuration.
- Order of negativity: Cl > F > Br > I >At
- Chlorine has more negative value than fluorine due to less repulsion in 3p orbital.
- Down the group: negativity decreases (size ↑, nuclear attraction ↓).
- Noble gases: positive ΔegH.
- Fluorine = strongest oxidising agent despite less negative ΔegH (due to weak F–F bond + high hydration enthalpy).
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