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 the main - group elements, when moving from group 2 to group 13 across a period, the first - ionization energy shows an exception. In group 2, the electron configuration has a full s - orbital ($ns^{2}$). When moving to group 13, an electron enters the p - orbital ($ns^{2}np^{1}$). The full s - orbital shields the electron in the p - orbital, making it easier to remove the p - orbital electron, so the ionization energy decreases.

Answer:

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