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?

First ionization energy generally increases across a period, but it decreases from group 2 to group 13. The group 13 atom’s first p electron is higher in energy and is shielded by the filled s subshell, so it is easier to remove than a group 2 s electron. Thus, the best explanation is that the full s orbital shields the electron entering the p orbital.

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