Outer electron shell

Figure Bl.24.14. A schematic diagram of x-ray generation by energetic particle excitation, (a) A beam of energetic ions is used to eject inner-shell electrons from atoms in a sample, (b) These vacancies are filled by outer-shell electrons and the electrons make a transition in energy in moving from one level to another this energy is released in the fomi of characteristic x-rays, the energy of which identifies that particular atom. The x-rays that are emitted from the sample are measured witli an energy dispersive detector.

With all these advantages one might well wonder why the left-step table has not attracted more attention and indeed why it has not been widely adopted. The answer to this question lies in the placement of one crucial element, helium. In the left-step table, helium is placed among the alkaline earth metals as mentioned above. To most chemists this is completely abhorrent since helium is regarded as the noble gas par excellence. Meanwhile, to a physicist or somebody who emphasizes electronic properties, helium falls rather naturally into the alkaline earths since it has two outer-shell 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]). 

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. 

Let us start at an elementary level or with a typically "chemical" view. Suppose we ask an undergraduate chemistry student how quantum mechanics explains the periodic table. If the student has been going to classes and reading her book she will respond that the number of outer-shell electrons determines, broadly speaking, which elements share a common group in the periodic table. The student might possibly also add that the number of outer-shell electrons causes elements to behave in a particular manner. 

Consequently, they maintain that some displays of the periodic system may, in truth, be superior to others. Whereas the conventionally displayed table, called the medium-long form, has many virtues, it places helium among the noble-gas elements. Some have argued that in spite of appearances, helium should in fact be placed el the head of group 2, the alkaline earth group, which includes beryllium, magnesium and calcium. Helium has two outer-shell electrons as do the elements in the alkaline earth group. 

The discussion in the previous section indicated that Nett values would be expected to exceed substantially the actual number of outer shell electrons. Table IV amply confirms this conclusion. Since N appears to the power in the Slater-Kirkwood equation, the deviations are exaggerated. Thus, in making a very similar treatment a few years ago in which slightly different empirical potentials were used, the writer31 found substantially smaller effective N values. For the same reason a relatively crude effective... 

The question now is, what role do the K, L, M,. . . electrons play in generating the K, L, M,. . . series The answer is not obviously predictable from a knowledge of visible or ultraviolet spectra. Neither hydrogen nor helium has a K series, although each has K electrons. Why Because the K series is generated only when the K shell contains a hole that is filled by an electron that leaves one of the outer (L, M,. . . ) shells or the generation of the K series requires (1) the absence of a K electron, (2) the presence of an outer-shell electron whose transition to the K shell is permitted by the selection rules. This picture explains why—no matter what the method of excitation—all K lines have the same excitation threshold so that all K lines appear together if they appear at all. 

Electrons that are in filled sets of orbitals between the nucleus and outer shell electrons shield the outer shell electrons partially from the effect of the protons in the nucleus this effect is called nuclear shielding. 

As we move from left to right along a period, the outer shell electrons do experience a progressively stronger force of attraction to the nucleus due to the combination of an increase in the number of protons and a constant nuclear shielding by inner electrons. As a result the atomic radii decrease. 

Metals are located at the left side of the periodic table and therefore, in comparison with nonmetals, have (a) fewer outer shell electrons, (b) lower electronegativities, (c) more negative standard reduction potentials and (d) less endothermic ionization energies. 

This is a free radical substitution reaction. Because chlorine atoms have 7 outer shell electrons, each will possess an unpaired electron. So 2 chlorine radicals are produced. A radical is a species that has a single unpaired electron. 

Square-planar stereochemistry is mostly confined to the d8 transition metal ions. The most investigated solvent exchange reactions are those on Pd2+ and Pt2+ metal centers and the mechanistic picture is well established (Table XIV (194-203)). The vast majority of solvent exchange reactions on square-planar complexes undergo an a-activated mechanism. This is most probably a consequence of the coordinatively unsaturated four-coordinate 16 outer-shell electron complex achieving noble gas... 

Intermolecular interactions only operate over relatively short distances, so we assume that, under normal conditions, each molecule in a gas is wholly unaffected by all the others. By contrast, when the gas is compressed and the particles come to within two or three molecular diameters of each other, they start to notice each other. We say the outer-shell electrons on an atom are perturbed by the charges of the electrons on adjacent atoms, causing an interaction. We call these interactions bonds, even though they may be too weak to be formal bonds such as those permanently connecting the atoms or ions in a molecule. 

Strictly, the bonds are held together with outer-shell electrons. 

Visible Excitation of valence and outer-shell electrons UV-visible spectroscopy... 

Near UY/visible 4-7.5 x 10 7 Valence (outer) shell electrons... 

Soft Low-charge density Large ionic radius Easily excited outer shell electrons Cu+ High polarizability Low electronegativity Low-energy vacant orbitals Easily oxidized RSH, RS-, CN", CO... 

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