Among the elements of the main group, the general trend in the first ionization energy moving across a period is not followed between group 2 and group 13. Which best explains these exceptions? The ionization energy decreases because the full s orbital shields the electron entering the p orbital. The ionization energy increases because the full s orbital shields the electron entering the p orbital. The ionization energy decreases because the stability of the half-full p subshell is increased. The ionization energy increases because the stability of the half-full p subshell is increased.