Aqua Ions and Related Problems

The second category of aqua ions corresponds to a mixture of differing N values (and symmetries) with almost the same free energy, readily modified by changing salt concentration, temperature (and perhaps even hydrostatic pressure). The rather indifferent choice between N = 7, 8, 9 and 10 may be characteristic for calcium(II), the rare earths and probably also for thorium(IV). The third category are cations with so large radii and so low positive charge (probably K+, Rb+, Cs+ and Ba+2, perhaps even Tl+) that the nearest water molecules are so relatively weakly perturbed and oriented by the cation that the situation is similar to the second layer of water molecules around Co(NH3) 3 or Cr(OH2)g 3, or even N(CH3)4.  

However, it is a quite striking fourth parameter that oxidizing aqua ions under equal circumstances are more acidic (have lower pK). Thus, iron(III) is distinctly more acidic than aluminium(III) and chromium(III), copper(II) more acidic than nickel(II) and zinc(II), and quite excessive acidity is observed for mercury(II)41), palladium(II)42,81) and thallium(III) aqua ions, compared with the much smaller beryllium(II) and aluminium(III). This tendency takes extreme proportions 841 in gold(III) complexes. 

Perhaps very fortunately, inorganic chemistry does not have one universal well-ordered deductive theory. An interesting case of the several complementary net-works of concepts used when rationalizing chemical facts can be found in the book92) by Williams. A mathematician may think about such a book as written in the language doublespeak of Orwell s novel 1984 . However, chemistry is much more inductive 10) than deductive, the facts are stubborn, and we do not yet know the surprises of tomorrow. 

7 Special Extensions of the Lewis Paradigm the Hybridization and the Kimball Models 

A model based on early ideas of Lewis was proposed by Kimball, enclosing each pair of electrons in a sphere. The atom or molecule consist of spheres touching each other, and containing the nucleus in the smallest two-electron sphere of a given atom. Like in quantum mechanics, the kinetic energy of each electron confined in a sphere with radius R is proportional to R-2 and the proportionality constant is chosen by 

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