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

Group 2 and group 13 do not follow these rules because in their case, s orbitals block electrons from going into the p orbital. So A is the answer.

Answer 2

The Ionization energy decreases because the full s orbital shields the electron entering the p orbital. This is known as "shielding", When each new electron experiences attraction from the nucleus and repulsion forces from the S orbitals, the net force on outer shell electrons is significantly smaller. Therefore, ionization energy decreases during these groups.