As the elements in a group 1 of the periodic table are considered from top to the bottom each successive element has a

Decreasing Ionization energy

Explanation:

As elements of Group 1 of the Periodic Table are considered in order from top to bottom, the ionization energy of each successive element decreases. This decrease is due to increasing radius and increasing shielding effect. In other words when we move to the right of a period, the number of electrons increases and the strength of shielding increases. As a result, it is easier for valence shell electrons to ionize, and thus the ionization energy decreases down a group.