Ionization energy is the amount of energy required to remove an electron from a neutral gaseous atom. The trend of ionization energy decreases down the group due to an increase in atomic size and increases from left to right in periods due to the decrease in atomic size. But, the ionization enthalpy of nitrogen is more than oxygen due to the half-filled stability of p-orbitals. Therefore, the correct order is F > N > O > S . Hence, sulfur has the lowest ionization energy.
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