Periodic table shells

According to the periodic table shells offer space for 2, 8, 8, 18, 18, and 32 electrons with increasing main quantum number. A valence shell of a small atom can contain only a small number of electrons larger atoms can have more electrons in their outer shells (18 in the fourth or fifth period or even 32 in the sixth period) in accordance with the aufbau principle of the periodic table. 

In any group of the periodic table we have already noted that the number of electrons in the outermost shell is the same for each element and the ionisation energy falls as the group is descended. This immediately predicts two likely properties of the elements in a group (a) their general similarity and (b) the trend towards metallic behaviour as the group is descended. We shall see that these predicted properties are borne out when we study the individual groups. 

If IS offen convenienf to speak of the valence electrons of an atom These are the outermost electrons the ones most likely to be involved m chemical bonding and reac tions For second row elements these are the 2s and 2p electrons Because four orbitals (2s 2p 2py 2pf) are involved the maximum number of electrons m the valence shell of any second row element is 8 Neon with all its 2s and 2p orbitals doubly occupied has eight valence electrons and completes the second row of the periodic table... 

Elements at the right of the periodic table tend to gam electrons to reach the elec tron configuration of the next higher noble gas Adding an electron to chlorine for exam pie gives the anion Cl which has the same closed shell electron configuration as the noble gas argon... 

Lead (qv) is a member of Group 14 (IVA) of the Periodic Table because it has four electrons in its outer, or valence, shell. However, the usual valence of lead is +2, rather than +4. The two s electrons have higher ionisation energies. As a result, tetravalent lead exists as a free, positive ion only in minimal concentrations. Furthermore, the bivalent or plumbous ion differs from the other Group 14 bivalent ions, such as the starmous ion of tin, because Pb " does not have reducing properties. 

Cadmium is a member of Group 12 (Zn, Cd, Hg) of the Periodic Table, having a filled d shell of electrons which dictates the usual valence state of... 

In addition to qualitative analysis of nearly all the elements of the periodic table, EEL spectra also enable determination of the concentration of a single element which is part of the transmitted volume and hence gives rise to a corresponding ionization edge. As in all comparable spectroscopic techniques, for quantification the net edge signal, which is related to the number N of excited atoms, must be extracted from the raw data measured. The net intensity 4 of the feth ionization shell of an individual element is directly connected to this number, N, multiplied by the partial cross-section of ionization ) and the intensity Iq of the incident electron beam, i.e. ... 

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... 

The three series of elements arising from the filling of the 3d, 4d and 5d shells, and situated in the periodic table following the alkaline earth metals, are commonly described as transition elements , though this term is sometimes also extended to include the lanthanide and actinide (or inner transition) elements. They exhibit a number of characteristic properties which together distinguish them from other groups of elements ... 

Like Rh and Ir, all three members of this triad have the fee structure predicted by band theory calculations for elements with nearly filled d shells. Also in this region of the periodic table, densities and mps are decreasing with increase in Z across the table thus, although by comparison... 

A contraction resulting from the filling of the 4f electron shell is of course not exceptional. Similar contractions occur in each row of the periodic table and, in the d block for instance, the ionic radii decrease by 20.5 pm from Sc to Cu , and by 15 pm from Y to Ag . The importance of the lanthanide contraction arises from its consequences ... 

I Nucleophilicity usually increases going down a column of the periodic table. Thus, HS- is more nucleophilic than HO-, and the halide reactivity order is I- > Br- > Cl-. Going down the periodic table, elements have their valence electrons in successively larger shells where they are successively farther from the nucleus, less tightly held, and consequently more reactive. The matter is complex, though, and the nucleophilicity order can change depending on the solvent. 

Paper four first appeared in the Journal of Chemical Education and aimed to highlight one of the important ways in which the periodic table is not fully explained by quantum mechanics. The orbital model and the four quantum number description of electrons, as described earlier, is generally taken as the explanation of the periodic table but there is an important and often neglected limitation in this explanation. This is the fact that the possible combinations of four quantum numbers, which are strictly deduced from the theory, explain the closing of electron shells but not the closing of the periods. That is to say the deductive explanation only shows why successive electron shells can contain 2, 8, 18 and 32 electrons respectively. 

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. 

Figure 10. Electron shell closure fails to coincide with the dosing of periods in the periodic table because the shells do not fill in strictly sequential order As shown here, die fourth shell begins to fill before the third shell has been completed. The resumption of third-shHl filling accounts for the appearance of the first transition-metal series, beginning with scandium and ending with zinc.

However, Pauli s Nobel Prize-winning work did not provide a solution to the question which I shall call the closing of the periods —that is why the periods end, in the sense of achieving a full-shell configuration, at atomic numbers 2,10, 18, 36, 54, and so forth. This is a separate question from the closing of the shells. For example, if the shells were to fill sequentially, Pauli s scheme would predict that the second period should end with element number 28 or nickel, which of course it does not. Now, this feature is important in chemical education since it implies that quantum mechanics cannot strictly predict where chemical properties recur in the periodic table. It would seem that quantum mechanics does not fully explain the single most important aspect of the periodic table as far as general chemistry is concerned. 

By adopting a perspective from the philosophy of science I will attempt to cross levels of complexity from the most elementary chemical explanations based on electron shells to those based on ab initio methods. Such a juxtaposition is seldom contemplated in the chemical literature. Textbooks provide elementary explanations which necessarily distort the full details but allow for a more conceptual or qualitative grasp of the main ideas. Meanwhile the research literature focuses on the minute details of particular methods or particular chemical systems and does not typically examine the kind of explanation that is being provided. To give a satisfactory discussion of explanation in the context of the periodic table we need to consider both elementary and deeper explanations within a common framework. 

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. 

But let me return to the question of whether the periodic table is fully and deductively explained by quantum mechanics. In the usually encountered explanation one assumes that at certain places in the periodic table unexpected orbital begins to fill as in the case of potassium and calcium where the 4s orbital begins to fill before the 3d shell has been completely filled. This information itself is not derived from first principles. It is justified post facto and by some tricky calculations (Melrose, Scerri, 1996 Vanquickenbome, Pierloot, Devoghel, 1994). 

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