What trend is observed in ionisation energy as you move down a group in the periodic table?

As you progress down a group in the periodic table, ionisation energy experiences a decrease primarily because of increased shielding. As additional electron shells are added with each subsequent element in a group, the inner electrons repel the valence electrons and effectively shield them from the nucleus's positive charge.

This shielding effect means that the outermost electrons are less tightly held by the nucleus, making it easier for the atom to lose these electrons. Consequently, the energy required to remove an electron from an atom (ionisation energy) decreases.

The reasoning behind the other options is that while some might suggest ionisation energy increases due to factors like atomic size or decreased atomic size, these factors do not accurately represent the trend observed. Indeed, as atomic size generally increases (more electron shells), the outer electrons are further from the nucleus, significantly impacting ionisation energy. Therefore, the correct understanding is that ionisation energy decreases as you go down a group due to increased shielding effects.