why does inoinzation energy tend to decrease from top to bottom within a group?

i'm not sure, but i think it has something to do with the size of the element

The electrons involved in ionization, that is, the outermost electrons, are farther away from the nucleus, and are held less strongly. The energy to remove them (ionization energy) is less.

You're on the right track! The decrease in ionization energy from top to bottom within a group is indeed related to the size of the elements.

Ionization energy is defined as the amount of energy required to remove an electron from an atom or ion in the gas phase. Within a group (also known as a family), the number of electron shells increases as you move down the periodic table. This means that the distance between the outermost electrons and the nucleus becomes greater.

The outermost electrons, also known as valence electrons, experience less attractive force from the positively charged nucleus because they are shielded by inner electron shells. As a result, these electrons are easier to remove, requiring less energy.

In other words, as you move down a group, the increasing number of electron shells acts as a barrier, reducing the attraction of the positively charged nucleus on the outermost electrons. Consequently, it becomes easier to remove these electrons, resulting in a decrease in ionization energy.

To summarize, the decrease in ionization energy within a group is primarily attributed to the increasing distance between the outermost electrons and the nucleus.