Aufbau principle and

Much of quantum chemistry attempts to make more quantitative these aspects of chemists view of the periodic table and of atomic valence and structure. By starting from first principles and treating atomic and molecular states as solutions of a so-called Schrodinger equation, quantum chemistry seeks to determine what underlies the empirical quantum numbers, orbitals, the aufbau principle and the concept of valence used by spectroscopists and chemists, in some cases, even prior to the advent of quantum mechanics. 

LOWDIN S REMARKS ON THE AUFBAU PRINCIPLE AND A PHILOSOPHER S VIEW OF AB INITIO QUANTUM CHEMISTRY. 

The video clip at www. brightredbooks.net will help you understand the Pauli exclusion principle, the aufbau principle and Hund s rule. 

The aufbau principle and the filling of orbitals is shown at www.brightredbooks.net... 

In many-electron atoms, the Schrodinger equation cannot be solved exactly, so approximations must be made. The simplest and crudest approximation is to neglect entirely electron-electron interactions (repulsions) and electron spin. In this way, hydrogenic orbitals are found as solutions. Into these orbitals we then place the electrons, according to the aufbau principle, and thus derive electron... 

With the aid of the aufbau principle and Hund s rule, we are now in position to determine the ground electronic configuration of a given octahedral complex. As shown in Fig. 8.2.1 there is only one electronic assignment for d -d3 and d8 d10 complexes, which have the ground configurations... 

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 -electrons over the tt-MOs is regulated by the Aufbau principle and the Pauli rule. The highest energy occupied MO thereof is called the HOMO (highest occupied molecular orbital). The lowest unoccupied MO thereof is called the LUMO (lowest unoccu-... 

In Appendix B, you will find a number of exceptions to the electron configurations predicted from the Aufbau Principle. Y)u should realize that statements such as the Aufbau Principle and the (w + 1) rule merely represent general guidelines and should... 

State the Aufbau principle and explain the role it plays in classifying the elements in the periodic table. 

Case (iii) is actually the most interesting, because it demonstrates most clearly the connection between breakdown of the aufbau principle and the appearance of many-body effects excitation of one electron to an extended orbital in V, Co and Ni must be accompanied by a rearrangement of the parent ion core (cf section 7.3). In other words, for these three elements, one cannot specify which electron (3d or 4s) should be removed first. Whichever is chosen, the result would be to leave the ion in an excited state. 

The orbitals with lowest energy level are filled first. The following examples sketch the electron configuration for all elements. That way the aufbau principle and construction of the periodic table will be clear. 

The multi-electron atom, the aufbau principle and electronic configurations... 

The eight valence electrons in H2O occupy the MOs according to the aufbau principle, and this gives rise to two occupied H—O—H bonding MOs and two occupied MOs with mainly oxygen character. (To appreciate this fully, see end of chapter problem 4.12.) Although this bonding model for H2O is approximate, it is qualitatively adequate for most descriptive purposes. 

---