## Chemistry: The Central Science (13th Edition)

The difference between the third and fourth ionization energies of $Sc$ is much larger than that of $Ti$ because: - In $Sc$, the third ionization still removes an outer electron, but the fourth ionization removes an inner electron. - In $Ti$, both the third and fourth ionization remove outer electrons.
Scandium $(Sc)$ has 3 electrons in the outermost shell $(n=4)$, while titanium $(Ti)$ has 4 electrons in the outermost shell. After the third ionization process, all 3 outermost electrons of $Sc$ would have been removed. So the fourth ionization would remove an inner electron of $Sc$. Since the inner electrons are much more strongly attached to the nucleus than the outer ones, to remove an inner electron would require a much higher energy than to remove an outer one. That explains the extreme difference between the third and fourth ionization energies of $Sc$. $Ti$, on the other hand, after the third ionization process, only 3 out of 4 outermost electrons have been removed. So the fourth ionization would remove the last outer electron of $Ti$. Since both the third and fourth ionization only remove outer electrons, the difference in energies between them is not as extreme as the difference in energy between removing outer electrons and inner electrons. That is why the difference between the third and fourth ionization energies of $Sc$ is much larger than that of $Ti$.