Niobium has Highest ionization energy.
Explanation:
The next ionization energy removes an electron from the same electron shell, which increases "ionization energy" due to "increased net charge"of the ion from which the electron is being removed.
Let's compare each of the metals first ionization energies,
Rubidium has its first ionization energy as 403 kJ / mol.
Zirconium has its first ionization energy as 660 KJ / mol.
Niobium has its first ionization energy as 664 KJ / mol.
From the given data above, we can infer that Rubidium has the least Ionization energy whereas Niobium has the highest Ionization energy. We can give the increasing order of the given elements as follows-
Rubidium < Zirconium < Niobium
