Final answer:
Exceptions to the ionization energy trends can occur when moving across periods in the periodic table. In periods 1 and 2, there is a decrease in ionization energy between certain groups due to variations in electron configurations. For example, the added 3p electron in aluminum is easier to remove than the 3s electrons in magnesium.
Step-by-step explanation:
Some noticeable exceptions to the ionization energy trends can be observed when moving across periods in the periodic table. In periods 1 and 2, there is a decrease in ionization energy between groups 2 and 3, as well as between groups 5 and 6. This decrease can be explained by looking at the electron configurations of the atoms in question. For example, as you move from magnesium to aluminum, the added 3p electron in aluminum is slightly further from the nucleus and is therefore easier to remove compared to the 3s electrons. Another exception is seen between phosphorous and sulfur, where the added electron in sulfur is the first to be paired in the p subshell, leading to repulsion between the two electrons. This makes the sulfur atom slightly less stable and easier to remove. These exceptions show that electron configurations can affect the ionization energy trends.