Answer:
Fluorine
Explanation:
Fluorine has an atomic number of 9
Iron's is 26
Phosphorus = 15
Potassium = 19
Fluorine has the smallest number of protons, and as it is an atom, the number of protons = the number of electrons. As it has less electrons, it only has 2 shells, whereas all the other elements here have 3+ shells. This means it will have a smaller atomic radius than them.
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