Question

Which of the following element have highest radius La2+,Cr3+,U3+,Yb2+

Answer 1

Answer:

Ybpluc2 becauce Yb atomic number ic 70

Answer 2

Answer:

1.2.2 The lanthanoid contraction

The ionic radii decrease by ca. 20% from La3+ to Lu3+ (Table 1.1, values for 8-coordination18). It arises through inadequate screening of the nuclear charge by the f electrons. As the number of f electrons increases, there is an increase in the effective nuclear charge leading to a progressive reduction in the Ln3+ size. The effect is so pronounced that with Ho3+ and Er3+, the ionic radius has decreased to that of Y3+, the rare earth of the second transition metal series. Consequently, the chemistry of Y resembles that of the heavy lanthanoids and is associated with them in the mineral xenotime. Yttrium is termed a heavy rare earth because of these similarities. A consequence of the contraction is that there is an increase in the stability of complexes with a particular ligand from La3+ to Lu3+. Complexes of Y3+ fit in according to the ionic radius of that element. The stability sequence associated with the lanthanoid contraction forms the basis for the separation of the lanthanoids by ion exchange or solvent extraction. The lanthanoid contraction can have a major effect on coordination number/structure of rare earth complexes. For example, [LnCl3(thf)n] (thf = tetrahydrofuran) complexes decline in coordination number from 8 (Ln=La) to 6 (Ln=Lu) [22a]. However, such changes are not inevitable as all 18-crown-6 complexes of LnCl3 and Ln(NCS)3 are 9-coordinate.22b,c Thus, one of the intriguing aspects of lanthanoid coordination chemistry is how complexes of a given ligand respond to the contraction. Structure/coordination number breaks can occur anywhere in the 15 elements (La-Lu), even between La and Ce. Cases are known of a structural break where a complex of the borderline element can be isolated in two forms. An example relevant to corrosion inhibitors is that [Dy(cinn)3] (cinn = cinnamate) can be isolated as both a 9- and a 7-coordinate complex.23 The lanthanoid contraction also has effects far beyond the lanthanoid series. Thus, it is responsible for why mercury and its compounds are so different from zinc and cadmium to the point where zinc and cadmium are more similar to magnesium (Group 2) than mercury. Other groups in the Periodic Table show changes between the last two elements, although not as substantial as those between cadmium and mercury.