Electronic Structure of Isolated Atoms

Chapters 1 and 2 dealt primarily with the electronic structure of isolated atoms of the various elements. With the exception of the noble gases, however, it is the molecule, not the individual atom, that is the basic building block of materials in nature. Even elements that occur naturally in their pure state are generally found in molecular form. For example, the oxygen and nitrogen in the atmosphere are made up of the diatomic molecules O2 and N2, not discrete O and N atoms. In Chapter 3, we begin our study of molecules and the chemical bonds that hold them together. 

The electronic structure of isolated atoms was determined by quanmm chemical calculations by dementi and Roetti Clementi, E. Roetti, C. Atomic Data and Nuclear Data Tables 1974,14,177. 

The table "Electronic Structure of Isolated Atoms," unlike similar tables as usually published, includes some refinements and additions in accordance with work done in recent years in particular the electronic configuration of the terbium atom has been refined. The table also gives the electronic structures proposed for the elements with atomic numbers from 98 to 103. 

The Kossel model (146) of single-electron transitions to unoccupied states has been applied to the interpretation of the absorption-edge structure of isolated atoms (inert gases) as well as to molecules and solids, in which case use is made of band-model calculations, including the possible existence of quasi-stationary bound states as exciton states. Parratt (229), who has carried out the first careful analysis of the absorption spectrum of an inert gas, assumed that dipole selection rules govern the transition possibilities, with allowed transitions being Is - np. 

The characteristic lines observed in the absorption (and emission) spectra of nearly isolated atoms and ions due to transitions between quantum levels are extremely sharp. As a result, their wavelengths (photon energies) can be determined with great accuracy. The lines are characteristic of a particular atom or ion and can be used for identification purposes. Molecular spectra, while usually less sharp than atomic spectra, are also relatively sharp. Positions of spectral lines can be determined with sufficient accuracy to verify the electronic structure of the molecules. 

Accepting that the electronic structure of the metal clusters is in between the discreet electronic levels of the isolated atoms and the band structure of the metals, it is expectable that under a certain size the particle becomes nonmetallic. Indeed, theoretical estimations [102,105] suggest that the gap between the filled and empty electron states becomes comparable with the energy of the thermal excitations in clusters smaller than 50-100 atoms or 1 nm in size, where the particles start to behave as insulators. A... 

The discovery of the rare earth elements provide a long history of almost two hundred years of trial and error in the claims of element discovery starting before the time of Dalton s theory of the atom and determination of atomic weight values, Mendeleev s periodic table, the advent of optical spectroscopy, Bohr s theory of the electronic structure of atoms and Moseley s x-ray detection method for atomic number determination. The fact that the similarity in the chemical properties of the rare earth elements make them especially difficult to chemically isolate led to a situation where many mixtures of elements were being mistaken for elemental species. As a result, atomic weight values were not nearly as useful because the lack of separation meant that additional elements would still be present within an oxide and lead to inaccurate atomic weight values. Very pure rare earth samples did not become a reality until the mid twentieth century. 

Complexes 75 are remarkably stable at room temperature in the solid state and, when heated, they start to decompose only at about 130 °C (Cr) or 145 °C (W). Such a thermal stability is undoubtedly associated with their strongly dipolar nature, in which six possible ylide-type resonance forms contribute to the bonding (Fig. 12). As expected, analysis of the electronic structure of complex [W (=C=C=C=C=C=C=C(NMe2)2 (CO)5] by DPT methods showed that the LUMO is mostly localized on the odd carbon atoms of the chain, whereas the HOMO is on the even carbons. In accord with these electronic features, it was found that [W =C=C=C=C=C=C=C(NMe2)21(00)5] readily adds dimethylamine across the 05=05 bond, to give the isolable alkenyl-pentatetraenylidene derivative [W =C=C=C=C=C(NMe2)CH=C(NMe2)21(00)5] [69, 70]. 

As is well known, in crystalline solids there may be formed collective electron-excitation states called excitons.8182 Such states are excited only in media with periodic structure and are delocalized over a large volume of atoms (or molecules), their excitation energy being 0.1-0.5 eV lower than the energy of the electron states of isolated molecules that produced them. The nature and spectroscopy of exciton states have been thoroughly studied both experimentally and theoretically. In this section we will... 

To find something out about molecules and bonds between atoms, the energies of the exciting photons or electrons have to be reduced. In contrast to the inner electrons which occupy isolated atomic orbitals, the energies of the outer electronic structure are determined by valence or conduction bands. 

It is also useful to know the formal charge of an atom. The formal charge is the number of electrons in the isolated atom, minus the number of electrons assigned to the atom in the Lewis structure. For instance, in the cyanide ion carbon has a pair of non.bondin.g electrons and one electron from each bond in the triple bond for a total of five electrons. 

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