As the elements in group 18 are considered in order of increasing atomic number, the ionization energy of each successive element

1) decreases
2) increases
3) remains the same

Ionization energy is the quantity of energy that an isolated, gaseous atom in the ground electronic state must absorb to discharge an electron, resulting in a cation. For example : H(gas) ➡H+ + e- . As we go down the group 18, number of shells added increases, causing increase in atomic radii. As we down the group, van der waal force increases cuz size increases. This van der waal force is further classified into two : London dispersion (it is weak force) and dipole - dipole ( strong). These van der waal forces are weak forces. Hence as we move down the group 18, ionization energy decreases. :-D. Ping me if you have any questions. Good luck :-)