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?
A
The ionization energy decreases because the full s orbital shields the electron entering the p orbital.B
The ionization energy increases because the full s orbital shields the electron entering the p orbital.C
The ionization energy decreases because the stability of the half-full p subshell is increased.D
The ionization energy increases because the stability of the half-full p subshell is increased.