Hafnium chemical properties

Element 104, the first transactinide element, is expected to have chemical properties similar to those of hafnium. It would, for example, form a relatively volatile compound with chlorine (a tetrachloride). 

Study of the chemical properties of element 104 has confirmed that it is indeed homologous to hafnium as demanded by its position in the Periodic Table (20). Chemical studies have been made for element 105, showing some similarity to tantalum (25) no chemical studies have been made for elements 106—109. Such studies are very difficult because the longest-Hved isotope of 104 ( 104) has a half-Hfe of only about 1 min, of 105 ( 105) a half-Hfe of about 40 s, of 106 ( 106) a half-Hfe of about 1 s, and of elements 107—109 half-Hves in the range of milliseconds. 

Hafnium [7440-58-6] Hf, is in Group 4 (IVB) of the Periodic Table as are the lighter elements zirconium and titanium. Hafnium is a heavy gray-white metallic element never found free in nature. It is always found associated with the more plentiful zirconium. The two elements are almost identical in chemical behavior. This close similarity in chemical properties is related to the configuration of the valence electrons, and for zirconium and... 

Chemical Properties. Hafnium s normal stable valence is also its maximum valence of four. Hafnium exhibits coordination numbers of six. 

Hafnium tetrabromide [13777-22-5], HfBr, is very similar to the tetrachloride in both its physical and chemical properties. Hafnium tetraiodide [13777-23-6], Hfl, is produced by reaction of iodine with hafnium metal at 300°C or higher. At temperatures above 1200°C, the iodide dissociates to hafnium metal and iodine. These two reactions are the basis for the iodide-bar refining process. Hafnium iodide is reported to have three stable crystalline forms at 263—405°C (60). 

K. L. Komarek, ed.. Hafnium Physico-Chemical Properties of Its Compounds andEUhys, International Atomic Energy Agency, Vieima, 1981, pp. 11,13,14, 16. Covers tbermocbemical properties, phase diagrams, crystal stmcture, and density data on hafnium, hafnium compounds, and alloys. 

Zirconium and hafnium have very similar chemical properties, exhibit the same valences, and have similar ionic radii, ie, 0.074 mm for, 0.075 mm for (see Hafniumand hafnium compounds). Because of these similarities, their separation was difficult (37—40). Today, the separation of zirconium and hafnium by multistage counter-current Hquid—Hquid extraction is routine (41) (see Extraction, liquid—liquid). 

The isolation and identification of 4 radioactive elements in minute amounts took place at the turn of the century, and in each case the insight provided by the periodic classification into the predicted chemical properties of these elements proved invaluable. Marie Curie identified polonium in 1898 and, later in the same year working with Pierre Curie, isolated radium. Actinium followed in 1899 (A. Debierne) and the heaviest noble gas, radon, in 1900 (F. E. Dorn). Details will be found in later chapters which also recount the discoveries made in the present century of protactinium (O. Hahn and Lise Meitner, 1917), hafnium (D. Coster and G. von Hevesey, 1923), rhenium (W. Noddack, Ida Tacke and O. Berg, 1925), technetium (C. Perrier and E. Segre, 1937), francium (Marguerite Percy, 1939) and promethium (J. A. Marinsky, L. E. Glendenin and C. D. Coryell, 1945). 

Haber process, 22 792 Hackling, 77 292 Hackmanite, color, 7 338 Haemanthamine, 2 87 Hafnium (Hf), 73 78-97 26 637. See also Hafnium compounds analytical methods for, 73 87-88 chemical properties of, 73 80 economic aspects of, 73 86-87 health and safety factors related to, 73 88... 

The chemical properties of hafnium are very much similar to those of zirconium. In aqueous solutions, the metal exists in tetravalent state. The elec-... 

Due to its 5t/-6.v- electron configuration, hafnium forms tctravalent compounds readily, although the Ilf1 ion docs not exist as such In aqueous solution except at very low pH values, Ihe common cation being HfO lor Hf OH)i ) and many of the tctravalent compounds are partly covalent. There are also less stable Hf(lll) compounds, There is close similarity in chemical properties to those of zirconium due to the similar outer electron configuration (4identical ionic radii (ZrJ is 0.80 A) the relatively low value for Hf being due lo the Lanthanide contraction. 

The similarity in size causes a very close similarity in chemical properties hafnium and zirconium compounds occur together in nature and are very difficult to distinguish from each other, and other pairs of elements following zirconium and hafnium resemble each other more closely than is usual for two successive members of a family. 

Solvent extraction has proved to be the most effective method for the separation of zirconium and hafnium, which invariably occur in nature in close association, owing to their almost identical chemical properties. These metals have found considerable use in the nuclear-power industry on account of their unusually high (hafnium) and low (zirconium) neutron-capture cross-sections. It is evident that the mutual separation of the two metals must be of a high degree to make them suitable for such applications. Two different solvent-extraction processes are known to be used on a commercial scale in one process, zirconium is selectively extracted from nitrate media into TBP in the second process, hafnium is selectively extracted from thiocyanate solutions into methyl isobutyl ketone (MIBK). 

In this chapter we will review the synthesis, structural aspects, and basic chemical properties of formally divalent and trivalent titanium and zirconium metallocene complexes. We have restricted our coverage to the low-valent bis(rj-cyclopentadienyl) and related metallocenes metal halide complexes and organometallic mixed metal systems will not be discussed here. We have not attempted to present an exhaustive coverage of the field. Rather, our aim has been to describe critically and to evaluate the often confusing chemistry that has been reported for the reactive low-valent titanium and zirconium metallocenes. More general reviews (7) and a book (2) on the organometallic chemistry of titanium, zirconium, and hafnium have been published. 

Zirconium and hafnium possess nearly identical chemistry. No other pair of congeners has chemical properties so similar to each other. This is mostly a result of the metals possessing nearly identical atomic and ionic radii (1.45 and 1.44 A for Zr and Hf, and 0.86 and 0.85 A for Zi + and Hf +) as a resnlt of the lanthanide contraction. They are relatively electropositive, but less so than the group 3 metals. The metals themselves... 

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. 

Element 104. (eka-hafnium) is predicted to resemble its homolc hafnium (element 72) in its chemical properties. It is expected to be predominantly tetra-positive, both in aqueous solution and in its solid compounds, although it should exhibit sohd halides and perhaps aqueous ions of the +2 and +3 oxidation state as well. 

Hafnium (Hf, at. mass 178.49) is much the same as zirconium in chemical properties. It usually accompanies zirconium to the extent of 1.5-2%. The methods for separation and determination of zirconium outlined below, also apply for hafnium. 

All these new discoveries, of course, verified Seaborg s theory, and the transuranium elements, along with thorium, protactinium and uranium, are now called the actinide elements. They all fit in the Periodic Table between actinium and the element eka-hafnium. Eka-hafnium is the tentative name given to the undiscovered element with the atomic number 104 which lies directly below hafnium in the Periodic Table and which is expected to have chemical properties similar to those of hafnium. 

Zirconium and hafnium have very similar chemical properties, invariably occur together in nature, and are difficult to separate. Yet their absorption cross sections for thermal neutrons are very different ... 

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