WHY FOR ELECTRONIC CONFIG OF f BLOCK ELEMENTS, {(n-1)d 0-1} d has only 0-1 electrons? why can't it have 2 or more?
In this table one can observe that only one electron is filled in d-orbital in some of the elements and in some they do not contain any electron in d-orbital. This is because in some of the elements its is energetically favorable to move move the single 5d electron into 4f orbital in most of the case due to their almost equivalent energy. The reason Gd has 5d1 arrangement is that this leaves a half filled configuration of f-orbital level which increases the stability, now if more than one electron fills in d-orbital, then the half filled configuration is disturbed and stability decreases. whereas Lu has 5d1 because here f orbital is completely filled and again if more than one electron fills in d-orbital, then the fully filled configuration is disturbed and element gets destabilized .
