Transition elements lanthanide contraction

The effect of the lanthanide contraction on the metal and ionic radii of hafnium has already been mentioned. That these radii are virtually identical for zirconium and hafnium has the result that the ratio of their densities, like that of their atomic weights, is very close to Zr Hf = 1 2.0. Indeed, the densities, the transition temperatures and the neutron-absorbing abilities are the only common properties of these two elements which differ... 

The total lanthanide contraction is of a similar magnitude to the expansion found in passing from the first to the second transition series, and which might therefore have been expected to occur also in passing from second to third. The interpolation of the lanthanides in fact almost exactly cancels this anticipated increase with the result, noted in preceding chapters, that in each group of transition elements the second and third members have very similar sizes and properties. 

As we go from left to right across the transition metals in the periodic table, the metal atoms become smaller, much as in the lanthanide contraction (Section 2.6). Furthermore, the atoms of elements of the first transition series are smaller than those of corresponding members of the second and third. Consequently, interstitial carbides are particularly important for metals toward the lower left of the series, as with TiC, ZrC, TaC, and the extremely hard tungsten carbide WC, which is used industrially as an abrasive or cutting material of almost diamond like hardness. The parallel with trends in chemisorption (Section 6.1) will be apparent. 

The atomic radii of the second- and third-series transition elements from group 4B on are nearly identical, though we would expect an increase in size on adding an entire principal quantum shell of electrons. The small sizes of the third-series atoms are associated with what is called the lanthanide contraction, the general decrease in atomic radii of the /-block lanthanide elements between the second and third transition series (Figure 20.4). 

Of course, the fascinating steric features of lanthanide elements are most impressively expressed in the lanthanide contraction [76]. In textbooks, lanthanide contraction is often simply explained as being the phenomena responsible for similar chemical properties, in particular between the pairs of the d-transition metal homologues Zr/Hf, Nb/Ta and Mo/W. However, what does the contraction mean to the lanthanide elements (and compounds derived from them)... 

The chemistry of all Ln3+ ions is therefore very similar and differentiated only by the gradual contraction in radius associated with increasing nuclear charge. The lanthanide contraction is also important for the transition elements of the 5d series. 

The greater similarity between the elements in the Second and Third Transition Series compared to those in the First Series (Table 2.1) was formerly ascribed solely to the Lanthanide Contraction, caused by the failure of the 5d and 6s shells to occupy the expected space, because the 5/ electrons do not adequately shield them from the increasing nuclear charge, by reason of the disposition of their orbitals 5d and 6s electrons are therefore drawn... 

Radii. The filling of the 4f orbitals (as well as relativistic effects) through the lanthanide elements cause a steady contraction, called the lanthanide contraction (Section 19-1), in atomic and ionic sizes. Thus the expected size increases of elements of the third transition series relative to those of the second transition series, due to an increased number of electrons and the higher principal quantum numbers of the outer ones, are almost exactly offset, and there is in general little difference in atomic and ionic sizes between the two heavy atoms of a group, whereas the corresponding atoms... 

The lanthanide contraction has a knock-on effect in the elements in the 5d transition series. It would naturally be expected that the 5d elements would show a similar increase in size over the 4d transition elements to that which the 4d elements demonstrate over the 3d metals. However, it transpires that the lanthanide contraction cancels this out, almost exactly, and this has pronounced effects on the chemistry, e.g. Pd resembling Pt rather than Ni, Hf is extremely similar to Zr. 

Niobium (formerly called columbium) and tantalum are Transition Metals having a considerable affinity for oxygen donor groups they are thus called oxophilic see Oxophilic Character). They occur as mixed-metal oxides such as columbites (Fe/Mn)(Nb/Ta)206 and pyrochlore NaCaNb206p. Their discovery in minerals extends back to the beginning of the nineteenth century, when they were believed to be identical and called tantalum. Rose showed that at least two different elements were involved in the minerals, and named the second one niobium. Their separation was resolved around 1866, especially by Marignac. These metals often display similar chemical behavior as a result of nearly identical atomic radii (1.47 A) due to the lanthanide contraction see Periodic Table Trends in the Properties of the Elements)... 

Another effect of lanthanide contraction is that the third row of the d-block elements have only marginally larger atomic radii than the second transition series. For example, zirconium and hafnium, niobium and tantalum, or tungsten and molybdenum have similar ionic radii and chemical properties (Zr + 80 pm, Hf + 81 pm Nb + 70 pm, Ta + 73 pm Mo + 62 pm, W + 65 pm). These elements are also found in the same natural minerals and are difficult to separate. 

In addition to making the third-series transition metals smaller, the lanthanide contraction also makes them less reactive because the valence electrons are relatively close to the nucleus and less susceptible to chemical reactions. This accounts for the relative inertness—or nobility—of these metals, particularly gold and platinum. Moreover, the third-series transition metals are the densest known elements, having about the same atomic size as the second-series transition metals but twice the atomic weight. The densest element is iridium (Ir, Z = 77) at 22.65 g/cm. ... 

Statement (c) is incorrect the atomic volumes of the transition metals decrease steadily from left to right the lanthanide contraction also affects the atomic volumes of third-row transition elements so that their atomic volumes are almost identical to those found for the second-row transition metals directly above. As a consequence, transition metals are very dense. For example, osmium with density of 22.6 g/cm is one of the densest materials known. 

Lanthanide contraction effects the properties of post lanthanide elements (elements of third transition series). For example,... 

Atomic and Ionic Radii Due to lanthanide contraction atomic radii of elements just following (i.e., Hf and onwards) are close to the elements just above them in their respective groups. Hence these elements are very much similar in properties and often very difficult to separate, e.g., Zr and Hf, Nb and Ta, Mo and W, Ru, Rh, Pd and Os, Ir, Pt, etc. This is why second and third row transition elements are quite close in properties while properties of first and second row are not. 

Densities Because of lanthanide contraction, the atomic size of Hf and succeeding elements becomes very small and their densities are high. That is why density of third row transition elements are almost double to those of elements of second transition series. 

All the lanthanides have similar outer electronic configuration and display mainly + 3 oxidation state in their compounds, therefore, lanthanides have exceedingly similar chemical properties. Their similarity is much closer than that of ordinary transition elements because lanthanides differ mainly in the number of 4/electrons which are buried deep in the atoms of lanthanides and thus don t influence their properties. Moreover, due to lanthanide contraction there is a very small difference in the size of all the fifteen tri valent lanthanide ions. Thus, for all practical purposes, the size of these ions is almost identical which results in similar chemical properties of these elements. 

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