Answer to Question #17018 in Inorganic Chemistry for Samantha
Why is [Ca] 3d^3 is not a valid configuration?
The appearance of 4s-electrons in the presence of free 3d-orbitals due to screening core by dense 3s^23p^6-electron layer. Due to the repulsion from the layer of outer electrons for calcium realizes the [Ar] 4s^2-state. So 3d^3 is not a valid configuration. The valid configuration is [Ca] 4s^2.