It is apparent that the most significant chemical properties are due to the s and p orbital electrons. The filling of the d and particularly the f orbitals make far less difference. In both the noble gases and the alkali metals there is a gradual decrease in IE 1 with increasing atomic number. In the smaller atoms the electrons are closer to the nucleus and are thus held more firmly. The nuclear charge is shielded by the inner electrons in larger atoms.
Ionization energies are very much a function of the electronic structure of the atom.
Just as IE 1 increases to peaks at the noble gases, IE 2 values are highest for the alkali metals. This is because the removal of the first electron from an alkali metal leaves not only a smaller positive ion but also the filled sub-orbital of a noble gas.