Explanation:
Ionization energy is defined as the energy required to remove most loosely bound electron from a gaseous atom or ion.
When we move down a group then there occurs a decrease in ionization energy. This is because size of atom increases down the group and valence shell is away from the nucleus of the atom.
Due to this there will be less force of attraction between the nucleus and valence electrons. As a result, it becomes easier to remove the valence electron.
For example, Na, Li, Rb and K are all group 1 elements.
Rb being the largest in size will have low ionization energy whereas Li being the smallest will have highest ionization energy.
Therefore, given elements are arranged from highest to least ionization energy as follows.
Li > Na > K > Rb
