According to Aufbau's Rule, It must be 5d8 6s2.. Even if considering stability; In terms of exchange energy and pairing, 5d9 has only 1 free electron, and thus no exchanges are possible as all other 4 orbitals are fully filled;( Thus decreasing stability and increasing pairing energy ). Still, 5d8 6s2 seems more stable as more exchanges are viable in the d subshell and even 6s is fully filled.
The more general answer I have stumbled upon is that the orbitals have nearly the same energies so 1 electron jumps from 6s to 5d, but it seems to violate the fact that subshells seek to be half or completely filled, so either electron must not jump or both electrons must jump like in the case of Palladium. I am Ignoring the fact that 6s is left half-filled which decreases stability ( As the element seeks the most stable configuration, Using General concepts 5d8 6s2 seems more stable)
All over the internet, There are either 2 answers:
2)Solve Schrodinger Dirac's Equation
Can someone please explain this exception? (Maybe even correcting some of my errors above) Thanks in Advance!!