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Which of the following sequences correctly represents the electron gain enthalpy values with negative sign for F, Cl, Br and I (atomic numbers 9, 17, 35 and 53, respectively)?

1. F > Cl > Br > I
2. Cl > F > Br > I
3. Br > Cl > I > F
4. I > Br > Cl > F

Correct answer: Cl > F > Br > I

Solution

Chlorine has the highest electron gain enthalpy due to its smaller atomic size and greater effective nuclear charge compared to fluorine, which is less favorable for gaining an electron. As we move down the group from Cl to I, the electron gain enthalpy decreases because the atomic size increases, making it less energetically favorable to add an electron.

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