Ionic versus atomic radii

The following ionic radii (in angstroms) are estimated for the +2 ions of selected elements of the first transition-metal series, based on the structures of their oxides Ca (0.99), TP+(0.71), V +(0.64), Mn +(0.80), Fe +(0.75), Co +(0.72), Ni (0.69), Cu (0.71), Zn (0.74). Draw a graph of ionic radius versus atomic number in this series, and account for its shape. The oxides take the rock salt structure. Are these solids better described as high- or low-spin transition-metal complexes ... 

Information concealed in data can often be most rapidly comprehended from graphical displays. Owing to the alternation in abundance between adjacent even-Z and odd-Z elements simple plots of the type absolute abundance versus atomic number will often easily obscure small diflFerences in lanthanide distribution patterns. The method often used to remove the even-Z, odd-Z eflFect is to divide one distribution, element by element, by a known distribution, and plot the resulting ratios on a logarithmic scale against a linear scale of atomic number or ionic radius (5). If the two distributions are identical, all the ratios are the same and a horizontal line appears. Trends of diflFerences in the distributions appear as curves or sloped lines. 

Figures 5-7 present the lattice parameters of the RFeioM2 compounds versus atomic number of the lanthanides or the ionic radius (fig. 5, Si Buschow 1988a fig. 6, Mo Ermolenko et al. 1990 fig. 7, Re Gueramian et al. 1991). For comparison, the lattice parameters of the respective uranium compounds are included (Suski et al. 1989, Gueramian et al. 1991). We do not know whether the U ion corresponds to Pr(U ) or to Nd(U " ), so for the sake of the argument, following the precedent set in fig. 4, its compounds are located at the position of Nd. One can see that as a rule the lattice parameters of the uranium compounds are smaller than those of respective Nd or Pr compounds and this difference is more pronounced for a-parameters. The difference probably results mainly from the smaller atomic radius of the uranium (1.38 A). The observation is general, but one cannot discuss the absolute values because they strongly depend on the details of the preparation process and the small differences in stoichiometry. The lattice parameters presented in figs. 5-7 decrease monotonously with increasing of the atomic number in accord with the lanthanide contraction.

Both perchloric acid and sulfuric acid have an 0-H group, and the acidic proton is part of an 0-H unit for both acids. Any differences in bond strength may be due to differences in the sulfur versus the chlorine. The covalent radii of S is 102 pm and that of Cl is 99 pm, so there is minimal difference. If there is little difference in bond strength in the acids, there may be more subtle factors a simple expedient is to examine the conjugate bases. The hydrogen sulfate anion has an ionic radius of 221 ppm, whereas the perchlorate anion has an atomic radius of 225 ppm. There is little difference in the size of the anions. 

Figure 1 Plot of ionic radius versus atomic number for the trivalent lanthanide elements (La-Lu). Also shown are the ionic radii for trivalent Y and Sc, for the Eu + and Ce +, and for other selected cations. The regular decrease in the ionic radii of the trivalent lanthanides is part of the lanthanide contraction. Sc is much smaller than the other rare earth elements and more similar in size to Fe + and Mg +...

Use the Handbook of Chemistry and Physics or a suitable website to find the Crystal Ionic Radii of the Elements. Plot atomic number versus radius of the 2 + ions of the first -transition series. Is there a trend Is a minimum size formed during the transition from Sc to Zn Do the values for the second and third -transition series appear to form a similar minimum at about the same group number ... 

The ionic radius of the lanthanide atom decreases with increasing number of 4f electrons, which is well known as lanthanide contraction. A plot of the lattice constant of the lanthanide compound versus the atomic number of the lanthanide element shows a nearly straight line, except for some Ce, Sm, Eu, Tm and Yb compounds, where the lanthanides can take integral valencies different from 3 as in Ce + and Sm +, Eu ", Tm, Yb +. The valence change in these compounds is brought about by changing the constitution x (such as in Smi-jrL Be Kasaya et al. 1980) or by... 

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