Answer:
Li > K > Cs
Step-by-step explanation:
The elements given are all alkaline metals. The belong to the first group on the periodic table, the elements in this group are:
H
Li
Na
K
Rb
Cs
Fr
Ionization energy is the energy required to remove the most loosely bound electron, the valence electron within an atomic space.
Down a group, the ionization energy reduces drastically. This is because the atomic radius of the atoms becomes larger as we go down.
Therefore, elements up in the group binds their electrons with more energy compared to those ones below.