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Solution. Elements of Group 1(Alkali metals)  have low first ionization energy. This is due to their large size and their valence shell electronic configuration, alkali metal easily loses its electron to attain noble gas configuration. Group 1 elements include the alkali metals, Li (lithium), Na (sodium), K (potassium), Rb (rubidium), Cs (caesium), Fr (francium). Hence, the answer would be 3 elements. 
Note: On moving down a group the ionization energy decreases. Thus, Cs will have the least first ionization energy.
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