Effective atomic number concept

Earlandite structure, 6,849 Edge-coalesced icosahedra eleven-coordinate compounds, 1, 99 repulsion energy coefficients, 1,33,34 Edta — see Acetic acid, ethylenediaminetetra-Effective atomic number concept, 1,16 Effective bond length ratios non-bonding electron pairs, 1,37 Effective d-orbital set, 1,222 Egta — see Acetic acid,... 

The noble gases in each of the three long periods have eighteen valence electrons (i.e. (n-l)s2, (n-Dp, nd O) and extension of the effective atomic number concept, therefore, suggests that many stable coordination compounds will similarly possess 18 valence electrons. 

The first attempts to interpret Werner s views on an electronic basis were made in 1923 by Nevil Vincent Sidgwick (1873—1952) and Thomas Martin Lowry (1874—1936).103 Sidgwick s initial concern was to explain Werner s coordination number in terms of the sizes of the sub-groups of electrons in the Bohr atom.104 He soon developed the attempt to systematize coordination numbers into his concept of the effective atomic number (EAN).105 He considered ligands to be Lewis bases which donated electrons (usually one pair per ligand) to the metal ion, which thus behaves as a Lewis acid. Ions tend to add electrons by this process until the EAN (the sum of the electrons on the metal ion plus the electrons donated by the ligand) of the next noble gas is achieved. Today the EAN rule is of little theoretical importance. Although a number of elements obey it, there are many important stable exceptions. Nevertheless, it is extremely useful as a predictive rule in one area of coordination chemistry, that of metal carbonyls and nitrosyls. 

The effective atomic number is a convenient parameter for defining the X-rays attenuation properties of a complex medium as a biological tissue, and particularly for the calculation of dose in radiography. The concept of the effective atomic number is based on a proportional relation of the elemental cross-section per atom to Z where m depends on the process considered. For a specific interaction the atomic cross-section of an element is generally expressed as... 

For nearly a century, the Lewis valence theory [1] and the subsequent development of the effective atomic number (BAN) rule [2, 3] as well as the valence bond theory [4] have constituted the fundamental basis concepts used for rationalizing the structure and bonding in a tremendously large area of covalent chemistry [5]. However, there are families of compounds, which have been, at least in part, reluctant to stick to this conventional two-center/two-electron approach, in particular those in which hypervalency and/or hypercoordination are present. This is the case, of... 

In Chapter 2, a series of concepts and models for describing bonding in borane and metal cluster were analyzed. The most versatile of such descriptions appears to be those rules related with the Effective Atomic Number (EAN) and with the Skeleton Electron Pairs (SEP). As assumed in some of the rules described in Table 2.16, frequently main group element fragments are actually involved in cluster bonding so that they obey the same electron counting rules. 

The concept of the effective atomic number applies particularly well to carbonyl and nitrosyl compounds of the d-block elements. For example, the composition of mononuclear nickel(O) and iron(0) carbonyl complexes may be rationalized in terms of effective atomic number. To attain the krypton configuration, nickel (28 electrons) and iron (26 electrons) need to accept four and five electron pairs, respectively. Thus, [Ni(CO)4] and [FefCOls] are the predicted compositions, linear nitrosyls are three-electron donors and so binding of a [Co(CO)3l fragment (33 electrons) to a single NO ligand to form [Co(NO)(CO)3] would result in the krypton electron configuration. 

The new delightful book by Greenstein and Zajonc(9) contains several examples where the outcome of experiments was not what physicists expected. Careful analysis of the Schrddinger equation revealed what the intuitive argument had overlooked and showed that QM is correct. In Chapter 2, Photons , they tell the story that Einstein got the Nobel Prize in 1922 for the explaining the photoelectric effect with the concept of particle-like photons. In 1969 Crisp and Jaynes(IO) and Lamb and Scullyfl I) showed that the quantum nature of the photoelectric effect can be explained with a classical radiation field and a quantum description for the atom. Photons do exist, but they only show up when the EM field is in a state that is an eigenstate of the number operator, and they do not reveal themselves in the photoelectric effect. 

The concept of an atom s oxidation state see Oxidation Number) can provide fundamental information about the stmcture and reactivity of the compound in which the atom is found. In fact, it can be argued that oxidation states provided the basis for Medeleev s initial organization of the periodic table. For the main group elements, the relative stability of lower oxidation states within a given group increases as the atomic number increases. This trend in the periodic table see Periodic Table Trends in the Properties of the Elements) is generally attributable to the presence of an inert s pair see Inert Pair Effect) caused by relativistic effects see Relativistic Effects). 

It may be surprising to learn that Mendeleev s success rate in his predictions was only 50%. It is quite interesting to note that while the successfiil predictions contributed significantly to the general acceptance of the periodic table, the failures did not have the opposite effect. These unsuccessful predictions did not lack logic, but chemistry turned out to confirm different lines of thought later. One should remember that Mendeleev did not have any idea about the concept of atomic number, neither did he have any information about the electron stmcture of atoms. Today, these two things make the periodic table almost trivial. 

As we have seen, the electron configurations of the elements show a periodic variation with increasing atomic number. Consequently, there are also periodic variations in physical and chemical behavior. In this section and the next two, we will examine some physical properties of elements that are in the same group or period and additional properties that influence the chemical behavior of the elements. First, let s look at the concept of effective nuclear charge, which has a direct bearing on many atomic properties. 

---