The electron configuration for $C$ is The values from $I_1$ to $I_4$ represent loss of the electrons in the outer shell of the atom. Although the nuclear charge each subsequaent value from $I_1$ to $I_2$ increases, because removal of each electron reduces interactions between the remaining electrons so effective nuclear charge $.I_5$ and $I_6$ represent loss of the electrons. These electrons are much closer to the nucleus and experience the full nuclear charge $($they are not shielded$),$ so the values of $I_5$ and $I_6$ are significantly greater than $I_1-I_4*I_6$ is larger than $I_5$ because all interactions have been eliminated.