The Nth ionization energy is the energy required to remove one extra electron from an ion that has had (N-1) electrons already removed. It takes successively greater energies to strip more and more electrons from the atoms. This is because, as further negatively charged electrons are removed from the atom, it becomes progressively more positively charged and clings ever more tenaciously onto its remaining electrons with greater and greater attractive force.
Which group of elements has the highest nth order ionization energies is determined by whether the (N-1)th ion has a closed shell:
Group 18 elements with a closed shell, have the highest 1st ionization energies.
Group 1 elements with one s-electron, the alkali metals, have the highest 2nd.
Group 2 elements with two s-electrons, the alkali earths, have the highest 3rd.
Group 13 elements with two s- and one p-electron, (boron etc), have the highest 4th.
Group 14 elements with two s- and two p-electrons, (carbon etc), have the highest 5th.
Group 15 elements with two s- and three p-electrons, (nitrogen etc), have the highest 6th.
Group 16 elements with two s- and four p-electrons, (oxygen etc), have the highest 7th.
Group 17 elements with two s- and five p-electrons, the halogens, have the highest 8th.
Group 18 elements with a closed shell, the noble gases, have the highest 9th.
Group 1 elements with one s-electron, the alkali metals, have the highest 10th ionization energies.
It is possible to remove the inner electrons from atoms leaving the outer ones in place to produce a 'hollow' atom. This requires even greater energy as the inner electrons are well shielded from external forces by the outer electrons, and being closer to the nucleus, are attracted to it with greater force than the outer electrons. These hollow atoms are of great interest, since, being essentially 'inverted', the outer electrons can jump down to inner un-occupied orbits either spontaneously or by stimulated emission, and in the process produce X-rays. The elusive X-ray laser.
