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.

Explanation:

In main - group elements, when moving from group 2 to group 13 across a period, the first ionization energy decreases. Group 2 has a full s - orbital ($ns^2$). When moving to group 13 ($ns^2np^1$), the electron is added to a p - orbital. The full s - orbital shields the p - electron, making it easier to remove. So, the ionization energy decreases.

Answer:

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