The atomic number of the element from the following with lowest ionisation enthalpy is:

Question

The atomic number of the element from the following with lowest  ionisation enthalpy is:

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Strategy:
 Find the most metallic/electropositive element among the options. Metals at the bottom-left of the table have the lowest Ionization Enthalpy.

Step 1: Identify elements by Z
-  (Ge): Group 14 metalloid.
-  (Br): Group 17 halogen.
-  (K): Group 1 alkali metal, Period 4.
-  (Fr): Group 1 alkali metal, Period 7.

Step 2: Compare
Down Group 1,  decreases. Francium () is much larger and has much lower  than Potassium ().

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