Aufbau filling diagram

Sadly, there are a few exceptions to the tidy picture presented by the Aufbau filling diagram. Copper, chromium, and palladium are notable examples (see Chapter 22 for details). Without going into teeth-grinding detail, these exceptional electron configurations arise from situations where electrons get transferred from their proper, Aufbau-filled orbitals to create half-filled or entirely filled sets of d orbitals these half- and entirely filled states cire more stable than the states produced by pure Aufbau-based filling. 

One He atom has two electrons, so a He2 cation has three electrons. Following the aufbau process, two electrons fill the lower-energy cr 1 orbital, so the third must be placed in the antibonding crj orbital in either spin orientation. A shorthand form of the MO diagram appears at right. The bond order 1... 

Figure 7-1 illustrates the Aufbau principle diagrammatically. The orbitals begin filling from the bottom of the diagram (lowest energy) with two electrons maximum per individual sublevel (line on the diagram). 

The chart below shows electron configurations and partial orbital diagrams for the 18 elements of period 4. You would expect the filling pattern shown for potassium (Z = 19) through vanadium (Z = 23). However, an unexpected deviation from the pattern occurs with chromium (Z = 24). The same thing happens with copper (Z = 29). All other configurations for period 4 conform to the aufbau principle. 

The progression of orbitals from lowest to highest energy is predicted by an Aufbau diagram. This isn t always true. Some atoms possess electron configurations that deviate from the standard rules for filling orbitals from the ground up. For Aufbau s sake, why ... 

FIGURE 6.10 Correlation diagram for first-period diatomic molecules. Blue arrows indicate the electron filling for the H2 molecule. All of the atomic electrons are pooled and used to fill the molecular orbitals using the aufbau principle. In the molecules, electrons are no longer connected to any particular atom. 

Table 5.3 summarizes several features of the aufbau diagram. Although the aufbau principle describes the sequence in which orbitals are filled with electrons, it is important to know that atoms are not built up electron by electron. 

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