Electron shells outer shell configurations

Akeypart in the analysis and proposal was electron accountancy - on the basis of the usual propensity for the adoption of a noble gas outer shell configuration - and all of the proposed intermediates have 16- or 18-electron configurations except the metallocyclobutane (26) in pathway II which has 14. On this basis, one might expect that pathway I, the non-dissociative pathway, would predominate over pathway II. In the presence of a large excess of Cy3P, which was used only to simplify the kinetic analysis, this is clearly the case. However, under the... 

Electron configurations of the neutral atoms are complex and have both d and 5 electrons in outer shells. For example, in the 3d series most atoms have the configuration (3df(4s)2, where n increases from one to 10 chromium and copper are, however, exceptions with (3d)5(4s) and (3[Pg.265]

Figure 6.31 allow us to reexamine the concept of valence electrons. Notice, for example, that as we proceed from Cl ([Ne]3s 3p ) to Br ([Arj3d 4s 4p ) we add a complete subshell of 3d electrons to the electrons beyond the [Ar] core. Although the 3d electrons are outer-shell electrons, they are not involved in chemical bonding and are therefore not considered valence electrons. Thus, we consider only the 4s and 4p electrons of Br to be valence electrons. Similarly, if we compare the electron configurations of Ag (element 47) and Au (element 79), we see that Au has a completely full 4/ subsheU beyond its noble-gas core, but those 4/electrons are not involved in bonding. In general,/or representative elements we do not consider the electrons in completely filled d or f subshells to be valence electrons, and for transition elements we do not consider the electrons in a completely filled f subshell to be valence electrons. 

In chemical education, the main motivation for basing chemistry on electronic configurations seems to be that if one knows the number of outer shell electrons in any particular atom, one can predict its chemical properties (Cotton and Wilkinson [1966], Kotz and Purcell [1987]). 

Figure 5. Niels Bohr came up with the idea that the energy of orbiting electrons would be in discrete amounts, or quanta. This enabled him to successfully describe the hydrogen atom, with its single electron, In developing the remainder of his first table of electron configurations, however, Bohr clearly relied on chemical properties, rather than quantum theory, to assign electrons to shells. In this segment of his configuration table, one can see that Bohr adjusted the number of electrons in nitrogen s inner shell in order to make the outer shell, or the reactive shell, reflect the element s known trivalency.

So what are we to make of the daim that the periodic table has now been explained in terms of electronic configurations and the number of outer-shell electrons possessed by atoms of the elements Perhaps the best way to answer this question is to admit that the explanation is approximate and that a number of objections can be raised to it. 

The octet rule tells us that eight electrons fill the outer shell of an atom to give a noble-gas ns1ns(l valence-shell configuration. However, when the central atom in a molecule has empty d-orbitals, it may be able to accommodate 10, 12, or even more electrons. The electrons in such an expanded valence shell may be present as lone pairs or may be used by the central atom to form additional bonds. 

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