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Chemistry Question 58 – JEE-MAIN 2025

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The property/properties that show irregularity in first four elements of group-17 is/are : (A) Covalent radius (B) Electron affinity (C) Ionic radius (D) First ionization energy Choose the correct answer from the options given below :

Recall the general trends of atomic properties (radius, ionization energy, electron affinity) down a group in the periodic table.

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Ninja Strategy•Focus on Anomalies

Recall specific anomalies in periodic trends, particularly for the first element of a group, like the electron affinity of Fluorine.

● Video Walkthrough
Step 1: Analyze General Periodic Trends for Group 17✦ Active

For the first four elements of Group 17 (F, Cl, Br, I):

1. Covalent radius: Increases down the group (F < Cl < Br < I). This is a regular trend.

2. Ionic radius (X−): Increases down the group (F−<Cl−<Br−<I−). This is a regular trend.

3. First ionization energy: Decreases down the group (F > Cl > Br > I). This is a regular trend.

Step 2: Identify Irregularity in Electron Affinity○ Expand

Electron affinity generally decreases down a group. However, for Group 17, there is an irregularity: Chlorine (Cl) has a higher electron affinity than Fluorine (F).

Typical values (in kJ/mol): F (-328), Cl (-349), Br (-325), I (-295). The trend is F < Cl > Br > I, showing an irregularity at Fluorine.

💡 Teacher's Secret Hint

Remember that electron affinity values are usually reported as negative for exothermic processes. A more negative value means higher electron affinity.

Step 3: Explain the Anomaly○ Expand

The irregularity in electron affinity for Fluorine is due to its exceptionally small size. The incoming electron experiences significant electron-electron repulsion from the already existing electrons in the compact 2p subshell of Fluorine. This repulsion makes the addition of an electron less favorable (less exothermic) compared to Chlorine, where the 3p subshell is larger and can accommodate the incoming electron with less repulsion.

Therefore, only electron affinity shows irregularity among the given properties.

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