Atomic structure Aufbau Principle

The energy-ordering scheme (5-58) coupled with the Pauli or exclusion principle and Hund s rule leads us to a simple prescription for building up the electronic configurations of atoms. This aufbau principle is familiar to chemists and leads naturally to a correlation between electronic structure and the periodic table. The procedure is to place all the electrons of the atom into atomic orbitals, two to an orbital, starting at the... 

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. 

Before estabiishing the connection between atomic orbitals and the periodic table, we must first describe two additionai features of atomic structure the Pauli exclusion principle and the aufbau principle. 

Aufbau principle chem A description of the building up of the elements in which the structure of each in sequence is obtained by simultaneously adding one positive charge (proton) to the nucleus of the atom and one negative charge (electron) to an atomic orbital. auf.bau phn s3-pal)... 

The general structure of the Periodic Table, based on atomic orbital energies, the aufbau principle, the Pauli exclusion principle and Hund s rules. 

The common long form of the periodic chart (Fig. 211) may be considered a graphic portrayal of the rules of atomic structure given previously. The arrangement of the atoms follows naturally from the aufbau principle The various groups oT the chart may be classified as follows ... 

This approach yields the shell model of the atom in which, under the restrictions of the Pauli principle and according to the aufbau principle, the electrons i are placed in the spin-orbitals (r, ms). For example, the shell structure of the magnesium atom is sketched schematically in Fig. 1.1. 

Dalton s atoms were not perceived as possessing structure, but the discovery of electrons, and the distinctive phenomenon of radioactivity, inevitably generated interest in the way atoms were put together. Clearly, this had a bearing on the periodic table as any proposed structure must explain atomic weights.24"28 The Aufbau Principle, that each element possessed one more proton in the nucleus and one more electron in the outer shell than the preceding element, effectively systematized the periodic table29-30... 

Use the aufbau principle to predict electron configurations of atoms and ions and to acconnt for the structure of the periodic table (Section 5.3, Problems 15-24). 

We may continue to build up in this manner from boron (At. No. 5) to neon. The building up process is often referred to as the aufbau principle, whereby the structure of a particular atom is the same as the one before it in the Periodic Table plus one additional electron. 

In view of this result it is unlikely that any occupied MO will be degenerate with any unoccupied MO, in particular the familiar spherically constrained model of atomic structure in which (e.g.) all 3d AOs, whether occupied or not, have the same energy and radial function is likely to violate the aufbau principle. 

The Madelung Rule is also caUed the Aufbau principle (from German ""Aufbau meaning build-up, fabric, structure ). This describes electron configuration and the filling of atomic orbitals. 

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