Question

Why oxygen is slightly larger in size than nitrogen

Answer 1

When electrons are closer it means atomic radius is smaller. When electrons are farther away from nucleus atomic radius is also larger.Nitrogen, with the same number of shell as Oxygen and Fluorine, has fewer proton than these two elements. Therefore, Nitrogen has lower effective nuclear charge.

Answer 2

oxygen is slightly larger than size than nitrogen because atomic number of oxygen is 8 whereas atomic no of nitrogen is 7.
Usually oxygen should have small size than nitrogen due to increase in nuclear charge but this abnormality is due to half filled d orbital of nitrogen which suppress the electrons towards itself.