Exceptions to the Ionization Energy Trend The reason for the discrepancy is due to the electron configuration of these elements and Hund’s rule. For beryllium, the first ionization potential electron comes from the 2s orbital, although ionization of boron involves a 2p electron.Click to see full answer. Similarly one may ask, what are the exceptions to the ionization energy trend?The two exceptions from the general trend are the ionization energies of B lesser than Be and that of O less than N. My teacher told me the reason to both was that half filled and fully filled orbitals of N and Be are more stable and hence require more energy to pull off an electron.Additionally, what affects ionization energy? The ionization energy increases as each electron is removed. Ionization energies are dependent upon the atomic radius. The more electrons shielding the outer electron shell from the nucleus, the less energy required to expel an electron from said atom. The higher the shielding effect the lower the ionization energy. Subsequently, question is, why is oxygen an exception to ionization energy? Oxygen also has an unexpectedly low ionisation energy, less than that of nitrogen. This is due to an electron being added to an already half full orbital in oxygen, which results in electron electron repulsion, which will lower the ionisation energy.Are there any exceptions to the atomic radius trend?There is only one exception in the trend of atomic radi along the period. By normal trend atomic radius increases along a period however the atomic radius of noble gases is fgreter than the adjacent halogen atom.