Explaining the shapes of ionization energy graphs tends to be more of a problem at Standard Level than Higher Level. This is mainly because the best explanation involves sub-levels and although ionization energies provide evidence for sub-levels the explanation for the discontinuities in ionization energy are only really covered in the AHL sub-topic S3.1.7 (see S1.3 & S3.1 (AHL) Electron configuration). Even so, Higher...