Question

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.

Answer 1

"The ionization energy decreases because the full s orbital shields the electron entering the p orbital."

Explanation: Group 13's outer electron is in an np orbital, which is higher in energy and less tightly held than the filled ns electrons of group 2. The filled s electrons provide extra shielding (lowering the effective nuclear charge felt by the p electron), so it is easier to remove — hence the drop in first ionization energy.