Which atom has the highest first ionization energy?

The atom with the highest first ionization energy is fluorine. Ionization energy refers to the amount of energy required to remove the most loosely held electron from an isolated gaseous atom.

Fluorine has a very high ionization energy because it is located in the top right corner of the periodic table, where elements generally have stronger effective nuclear charges and smaller atomic radii. This means that the valence electrons are held more tightly due to the increased nuclear charge, making them harder to remove. As you move across a period from left to right, ionization energy increases due to the increasing number of protons in the nucleus without a significant increase in shielding from inner electrons.

In this specific question, nitrogen, oxygen, and chlorine all have lower first ionization energies compared to fluorine. While oxygen has a relatively high ionization energy, it is less than that of fluorine. Chlorine, being further down the periodic table than fluorine, also has a lower ionization energy because of its larger atomic radius and increased electron shielding. Thus, fluorine clearly ranks as the element with the highest first ionization energy among the four elements listed.