NEET 2026 ALL Q #1955
COMPETENCY BASED
APPLY
4 Marks 2026 NTA-RE-NEET-2026 MCQ SINGLE
Among the following options, the correct trend in the electron gain enthalpy is
(A) $I>Br>Cl>F$
(B) $F>Cl>Br>I$
(C) $Br>Cl>F>I$
(D) $Cl>F>Br>I$
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Correct Answer: D

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Detailed Solution

Step 1: Understanding Electron Gain Enthalpy

Electron gain enthalpy is the energy change when an electron is added to a neutral gaseous atom. Generally, it becomes more negative (more exothermic) as we move across a period and less negative as we move down a group due to increasing atomic size.

Step 2: Analyzing the Halogen Group

For the halogens (Group 17), the expected trend based on size would be $F > Cl > Br > I$ (in terms of magnitude of energy released). However, Fluorine has a very small atomic size, which leads to significant inter-electronic repulsions when an incoming electron is added to its compact 2p subshell.

Step 3: Comparing Chlorine and Fluorine

Due to the high electron-electron repulsion in the small 2p orbital of Fluorine, the energy released upon adding an electron to Chlorine is greater than that for Fluorine. Thus, the order is $Cl > F > Br > I$.

Final Answer: Cl > F > Br > I

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because the student must apply the concept of periodic trends while accounting for the specific anomaly caused by electron-electron repulsion in small atoms.
Knowledge Dimension: CONCEPTUAL
Justification: The question tests the understanding of the underlying principles of periodic properties rather than rote memorization of values.
Syllabus Audit: In the context of NEET, this is classified as COMPETENCY. This is a classic high-yield topic in Inorganic Chemistry (Classification of Elements and Periodicity in Properties) frequently tested in competitive exams.

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