Why does group 18 have the highest Ionization Energy?

Ionization Energy: The amount of energy required to remove an electron from a gaseous atom is known as ionization energy.

The ionization energy of group 18 :

• The general electronic configuration of group 18 element is ${\mathrm{ns}}^2{\mathrm{np}}^6$.
• So, the group 18 elements have a complete octet.
• Due to stable electronic configuration, it is difficult to remove an electron from fully filled orbital.
• Hence, the ionization energy of group 18 elements is very high.