The electron gain enthalpy (in kJ/mol) of fluorine, chlorine, bromine and iodine, respectively, are:

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Strategy:
 Recall the specific values or trends for halogen electron gain enthalpies (). The  period member (Chlorine) has a more negative value than the  period member (Fluorine).

Step 1: Apply the known values/trend
Down Group 17, the magnitude of energy released generally decreases, but Fluorine is an exception:
- Chlorine:  (Most exothermic)
- Fluorine:  (Less than Cl due to compact  repulsion)
- Bromine: 
- Iodine:

Order of magnitudes: .

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