Why is first ionization energy of nitrogen higher than that of oxygen?

The first ionization energy of an element is the energy required to remove the outermost electron from an atom in its gaseous state. In the case of nitrogen and oxygen, the first ionization energy of nitrogen is higher than that of oxygen because nitrogen has a smaller atomic size and a higher effective nuclear charge compared to oxygen.

The smaller atomic size of nitrogen means that the outermost electron is closer to the nucleus, making it more strongly attracted to the nucleus and more difficult to remove. Additionally, nitrogen has one less electron shell than oxygen, resulting in a higher effective nuclear charge that further increases the attraction between the nucleus and the outermost electron.

Overall, the combination of a smaller atomic size and a higher effective nuclear charge in nitrogen results in a higher first ionization energy compared to oxygen.