Aufbau diagram

Figure 3.30 The NBO Aufbau diagram for first-row homonuclear diatomics, showing expected NHO configurations (right-hand panels) for each species.

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 ... 

Flip to Chapter 4 for details on electron configurations and Aufbau diagrams. 

Writing what is called the electron configuration is a way of describing each of an element s electrons. It requires an Aufbau diagram (Figure 8.12) and knowledge of the number of electrons in a particular atom. The number of electrons is shown on the periodic table as the smaller of the two numbers in the square for each element. 

The aufbau diagram shows the energy of each sublevel. Each box on the diagram represents an atomic orbital. Does the 3d or 4s sublevel have greater energy ... 

Exceptions to predicted configurations You can use the aufbau diagram to write correct ground-state electron configurations for all elements up to and including vanadium, atomic number 23. However, if you were to proceed in this manner, your configurations for chromium, [Ar]4s 3d, and copper, [Ar]4s 3d, would prove to be incorrect. The correct configurations for these two elements are ... 

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. 

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... 

Recall from Chapter 8 that ionization energy refers to the removal of an electron from an atom, or, in this case, from a molecule. We must count the valence electrons, choose the correct MO diagram, follow the aufbau process in placing the electrons, and then use the configurations to explain the ionization energy data. 

Textbook discussions of homonuclear diatomic molecules are commonly based on the familiar type of MO energy diagram shown in Fig. 3.28, which underlies the standard MO Aufbau procedure for constructing many-electron molecular configurations (which is analogous to the well-known procedure for atoms). Figure 3.28 purports to represent the energies and compositions of available MOs, which are... 

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). 

Be able to write both the energy-level diagram and the electronic configuration of an atom or ion by applying both the Aufbau build-up principle and Hund s rule. 

Use the aufbau principle to write complete electron configurations and complete orbital diagrams for atoms of the following elements sodium, magnesium, aluminum, silicon, phosphorus, sulfur, chlorine, and argon (atomic numbers 11 through 18). 

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. 

For each of the elements below, use the aufbau principle to write the full and condensed electron configurations and draw partial orbital diagrams for the valence electrons of their atoms. You may consult the periodic table in Appendix C, or any other periodic table that omits electron configurations. 

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. 

Orbital diagrams assign electrons to individual orbitals so the energy state of individual electrons may be found. This requires knowledge of how electrons occupy orbitals within a subshell. Hund s rule states that before any two electrons occupy the same orbital, other orbitals in that subshell must first contain one electron each with parallel spins. Electrons with up and down spins are shown by half-arrows, and these are placed in lines of orbitals (represented as boxes or dashes) according to Hund s rule, the Aufbau principle, and the Pauli exclusion principle. Below is the orbital diagram for vanadium ... 

The distribution of 77 electrons over the 77 MOs is regulated by the Aufbau principle and the Pauli rule. The only occupied MO of an MO diagram or the highest occupied MO thereof is called the HOMO (highest occupied molecular orbital). The only unoccupied MO of an MO diagram or the lowest unoccupied MO thereof is called the LUMO (lowest unoccupied molecular orbital). HOMOs and LUMOs are the so-called frontier orbitals since they flank the borderline between occupied and unoccupied orbitals. 

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