Which of the following has highest second ionization energy?

Step-by-step solution

Electronic configuration for chromium is : ₂₄ Cr 1 s^2 2 s^2 2 p^6 3 s^2 3 p^6 3 d^5 4 s^1 After ionisation of 4 s electron the next electron which will be ionised is from 3 d orbital. As 3 d^5 electronic configuration, is stable, hence, the second ionisation of Cr will require much greater ionisation energy than the expected one.