Scandium orbital energies

The interesting part is what happens next. In the case of the following element, number 21, or scandium, the orbital energies have reversed so that the 3d orbital has a lower energy. Textbooks almost invariably claim that since the 4s orbital is already full there is no choice but to begin to occupy the 3d orbital. This pattern is supposed to continue across the first transition series of elements, apart from the elements Cr and Cu where further slight anomalies are believed to occur. 

The full explanation of why the 4s 3d configuration is adopted in scandium, even though the 3d level has a lower energy, emerges from the peculiarities of the way in which orbital energies are defined in the Hartree-Fock procedure. The details are tedious but have been worked out and I refer anyone who is interested in pursuing this aspect to the literature (Melrose, Scerri, 1996).6,7... 

Fig. 3.1. The atomic orbital energies associated with the ground state [Ar]4s 3d and excited state [Ar]4i 3d electron configurations of scandium. [1]...

When we add the next electron to form the element scandium, the orbital of lowest energy that is available is one of the 3d orbitals (since the 3d orbitals are slightly lower in energy than the 4p orbitals). As succeeding electrons are added to form other elements, they enter the 3d orbitals until the ten available spaces in these orbitals are filled. 

There are five 3d orbitals available, all more or less of the same energy. Putting a pair of electrons in each of these five orbitals means that a total of ten electrons can be accommodated before we need to go to a higher energy level. Not only scandium but the nine following elements can be built up by adding electrons into 3d orbitals. Not until we get to gallium (element number 31) do we go up to another set of orbitals. 

For any cation, the empty 4 S orbital is slightly higher in energy than the partially filled 3 d orbital. Thus, the isoelectronic V and Cr cations both have the [Ar] 3 d configuration. On the other hand, the isoelectronic neutral atom scandium has the configuration [Ar]4 3 d ... 

From scandium to zinc the 3d-shell begins to fill up until it is full with 10 electrons, then the next most favourable energy level starts to fill up, i.e. the 4p-orbital. 

The last electrons to be added to an orbital diagram for the atoms of the transition metal elements go into d orbitals. For example, the last electrons added to atoms of scandium. Sc, through zinc, Zn, are added to 5d orbitals. The elements yttrium, Y, through cadmium, Cd, have their highest-energy electrons in the Ad sublevel. The elements directly below them in rows 6 and 7 add electrons to the 5d and Gd orbitals. The transition metals can be called the block. (Figure 11.16). 

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