Exceptions in electron configurations

[Ali Muldoon 2D]

Why do Cu+ and Tl3+ have exceptions to the electron configuration rules (they apparently fill the d shell before the s shell)? I thought these exceptions were only for Chromium (Cr) and Copper (Cu)?

[Sean Cronin]

The more commonly known exceptions for electron configurations are definitely Cr and Cu. However, atoms such as Tl3+ have the ability to completely fill their d-orbital before the 6s or 6p orbitals, and this figuration is more stable than what would be expected based on the Aufbau Principle. So, this is just another exception to be aware of, as is Cu+, which follows the same concept as Cu, but lacks the 4s1 and is simply just [Ar]3d10.