Free-ion models

Errecalde, O., Seidl, M. and Campbell, P. G. C. (1998). Influence of a low molecular weight metabolite (citrate) on the toxicity of cadmium and zinc to the unicellular green alga Selenastrum capricornutum an exception to the free-ion model, Water Res., 32, 419 -29. 

Parent, L., Twiss, M. R. and Campbell, P. G. C. (1996). Influences of natural dissolved organic matter on the interaction of aluminum with the microalga Chlorella a test of the free-ion model of trace metal toxicity, Environ. Sci. Technol., 30, 1713-1720. 

CP Free ion model Calc. Expt. Free ion model Calc. Expt. 

Krishnamurti, G. S. R., Megharaj, M., and Naidu, R. 2004. Bioavailability of cadmium-organic complexes to soil alga an exception to the free ion model. J. Agric. Food Chem. 52, 3894-3899. 

Such anion-anion overlap causes a broadening of the Compton profile, and it has been shown that quite good agreement between experiment and a free ion model may be obtained for other rock-salt compounds if the effect is included, although it is always smaller than in LiH. Thus for LiF and LiCl< > the anion-anion overlap is calculated to have a 10% effect on the Compton profile at low momentum values but only a 1 % effect on x-ray scattering factors. As expected, MgO shows larger overlap effects than the halides. In other, more covalent compounds, for example BeO< > and A1N< >, the expected large departures from ionic model behavior have been observed but have yet to lead to improved wavefunctions for the solids. 

Such studies, which give direct information on the distribution of electrons in specific valence orbitals, have rarely been performed in the required detail to give quantitative information on bonding. The form factor of single crystal antiferromagnetic NiO< > showed an expansion which fitted closely with the free ion model including overlap and orbital effects. Indeed, the measurement of the covalently reduced moment and form factor expansion for this compound preceded the understanding of these effects. Early measurements on several polycrystalline Mn + compounds indicated a form factor contraction< > which was taken to be a reflection of the nephelauxetic effect—the expansion of the... 

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