Electron structures configurations

In practice, each CSF is a Slater determinant of molecular orbitals, which are divided into three types inactive (doubly occupied), virtual (unoccupied), and active (variable occupancy). The active orbitals are used to build up the various CSFs, and so introduce flexibility into the wave function by including configurations that can describe different situations. Approximate electronic-state wave functions are then provided by the eigenfunctions of the electronic Flamiltonian in the CSF basis. This contrasts to standard FIF theory in which only a single determinant is used, without active orbitals. The use of CSFs, gives the MCSCF wave function a structure that can be interpreted using chemical pictures of electronic configurations [229]. An interpretation in terms of valence bond sti uctures has also been developed, which is very useful for description of a chemical process (see the appendix in [230] and references cited therein). 

Information about the structure of a molecule can frequently be obtained from observations of its absorption spectrum. The positions of the absorption bands due to any molecule depend upon its atomic and electronic configuration. To a first approximation, the internal energy E oi a, molecule can be regarded as composed of additive contributions from the electronic motions within the molecule (Et), the vibrational motions of the constituent atoms relative to one another E ), and the rotational motion of the molecule as a whole (Ef) ... 

Structure determines properties and the properties of atoms depend on atomic struc ture All of an element s protons are m its nucleus but the element s electrons are dis tributed among orbitals of varying energy and distance from the nucleus More than any thing else we look at its electron configuration when we wish to understand how an element behaves The next section illustrates this with a brief review of ionic bonding... 

There is no single best form of the periodic table since the choice depends on the purpose for which the table is used. Some forms emphasize chemical relations and valence, whereas others stress the electronic configuration of the elements or the dependence of the periods on the shells and subshells of the atomic structure. The most convenient form for our purpose is the so-called long form with separate panels for the lanthanide and actinide elements (see inside front cover). There has been a lively debate during the past decade as to the best numbering system to be used for the individual... 

It can now be seen that there is a direct and simple correspondence between this description of electronic structure and the form of the periodic table. Hydrogen, with 1 proton and 1 electron, is the first element, and, in the ground state (i.e. the state of lowest energy) it has the electronic configuration ls with zero orbital angular momentum. Helium, 2 = 2, has the configuration Is, and this completes the first period since no... 

In so far as the chemical (and physical) properties of an element derive from its electronic configuration, and especially the configuration of its least tightly bound electrons, it follows that chemical periodicity and the form of the periodic table can be elegantly interpreted in terms of electronic structure. 

This is larger than the corresponding value for Li (57 kJ mol" ) but substantially smaller than the value for F (333kJmol" ). The hydride ion H" has the same electron configuration as helium but is much less stable because the single positive charge on the proton must now control the 2 electrons. The hydride ion is thus readily deformable and this constitutes a characteristic feature of its structural chemistry (see p. 66). 

In those few cases where hydration and pseudobase formation parallel each other, the agreement can be traced to the fortuitous circumstance that the structure and electronic configuration of the molecule permit both phenomena to occur simultaneously. Quin-azoline-3-methochloride, one of these rare examples, is discussed in Section III,C, 1. 

An atom consists of a positively charged nucleus surrounded by one or more negatively charged electrons. The electronic structure of an atom can be described by a quantum mechanical wave equation, in which electrons are considered to occupy orbitals around the nucleus. Different orbitals have different energy levels and different shapes. For example, s orbitals are spherical and p orbitals are dumbbell-shaped. The ground-state electron configuration of an... 

As pointed out in Chapter 2, elements close to a noble gas in the periodic table form ions that have the same number of electrons as the noble-gas atom. This means that these ions have noble-gas electron configurations. Thus the three elements preceding neon (N, O, and F) and the three elements following neon (Na, Mg, and Al) all form ions with the neon configuration, is22s22p6. The three nonmetal atoms achieve this structure by gaining electrons to form anions ... 

To illustrate these rules, consider the Fe2+ ion. Because the atomic number of iron is 26, this +2 ion must contain 26 — 2 = 24e. Of these electrons, the first 18 have the argon structure the remaining six are located in the 3d sublevel The abbreviated electron configuration is I... 

Beryl. 385 Beryllium atomic size, 379 boiling point, 374 bonding capacity, 285 chemistry of, 382 electron configuration. 378 heat of vaporization, 374 ionization energies, 379 occurrence, 384 preparation, 385 properties, 381 structure, 381... 

Stronlianile, 385 Strontium atomic size, 379 chemistry, 382 electron configuration. 378 heal of vaporization, 305 hydroxide, K,p, 383 ionization energies, 379 occurrence, 385 properties, 381 Structural formula, 31 Structural isomers, 327 Styrene, 345 Sublimation, 176 Substance, 28 pure, 29, 65, 70... 

A few studies are starting to claim correlations between nuclear structure and electronic configurations such as the occurrence of anomalous configurations in atoms [41-43]. 

This procedure gives the ground-state electron configuration of an atom. Any other arrangement corresponds to an excited state of the atom. Note that we can use the structure of the periodic table to predict the electron configurations of most elements once we realize which orbitals are being filled in each block of the periodic table (see Fig. 1.44). 

What Do We Need to Know Already This chapter assumes that we know about atomic structure and electron configurations (Chapter 1), the basic features of energy, and the nature of the Coulomb interaction between charges (Section A). It is also helpful to be familiar with the nomenclature of compounds (Section D) and oxidation numbers (Section K). 

---