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Chemistry Question 53 – NEET-UG 2026

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Among the following options, the correct trend in the electron gain enthalpy is

Electron gain enthalpy generally becomes more negative across a period and less negative down a group.

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Ninja Strategy•Halogen Anomaly Recall

Immediately recall the anomalous trend for fluorine and chlorine, where chlorine has a more negative electron gain enthalpy than fluorine, then apply the general group trend.

Step 1: Understand Electron Gain Enthalpy Trend✦ Active

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

Step 2: Analyze Halogen Trend and Anomaly○ Expand

For halogens (F, Cl, Br, I), the general trend would suggest that F has the most negative electron gain enthalpy. However, due to the very small size of the fluorine atom, the added electron experiences significant electron-electron repulsion from the existing electrons in the compact 2p subshell. This makes the electron gain enthalpy of fluorine less negative than that of chlorine.

💡 Teacher's Secret Hint

Remember the anomalous behavior of fluorine compared to chlorine for electron gain enthalpy.

Step 3: Determine the Correct Order○ Expand

Considering the anomaly, the correct order of electron gain enthalpy (most negative to least negative, i.e., most exothermic to least exothermic) for halogens is Chlorine > Fluorine > Bromine > Iodine. This corresponds to option (3).

Cl>F>Br>I
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