Atom-at-a-time chemistry

This chapter is divided in four parts. The first one is devoted to thermodynamic considerations, the second deals with kinetic aspects and one-atom-at-a-time chemistry. Then, experimental approaches and finally effects of the media and their influence on aqueous chemistry will be discussed. 

The concept of one-atom-at-a-time chemistry is different from that of a single atom chemistry. The main difference is the time. An observable chemical reaction of a given radio nuclide can be defined with the following temporal quantities ... 

Problems associated with the disintegration of a single radioactive atom will not be developed here However, an understanding of the decay appears rather important in one-atom-at-a-time chemistry [2,10],... 

Since the SHE chemistry is correlated with one-atom-at-a-time chemistry, one may ask if it is meaningful to carry out experiments with a single atom. From a theoretical point of view, as it was demonstrated above, for a 1 1 stoichiometry reaction the mass action law and the kinetics laws are valid. However, as there is no macrocomponent consumption, such reaction appears as of pseudo first order. Note that reactions with a 1 1 stoichiometry include all reactions between the microcomponent and a single macrocomponent. This concept can also be extended to stepwise reactions such as successive formation of metal complexes (hydrolysis, halide complexation) for example ... 

The feasibility of Kd determination in the context of one-atom-at-a-time chemistry is very promising since the collection of Kd values will allow to establish reliable variations of the chemical properties (complexation,... 

D.C. Hoffmann and D.M. Lee (1999) Journal of Chemical Education, vol. 76, p. 331 - An excellent article that covers the development and future prospects of atom-at-a-time chemistry. 

The scale on which these transmutations is carried out is extremely small, and in some cases has been described as atom-at-a-time chemistry. The target materials in equations 3.22 and 3.23 are actinoid elements (see Chapter 25), which, although artificially prepared, have relatively long half-lives ( 9vBk, fi = 300 days geCm, h = 3.5 x 10 yr). Studying the product nuclides is extremely difficult, not only because of the tiny quantities of materials involved but also because of their short half-lives (losLr, t = 3 min lo Rf, ti = 65 s). 

In this chapter, the special conditions of the synthesis and decay of the transactinides are considered followed by some remarks about performing chemistry with only one atom at a time (referred to therefore as one-atom-at-a-time chemistry or single-atom chemistry). The experimental techniques are outlined, and a description of theoretical predictions and experimental results on the chemistry of Rf, Db, Sg, Bh, Hs, and Cn is given and mutually compared. Perspectives to go beyond 112 are also briefly discussed. 

The actinides uranium and thorium occur in nature as primordial matter. Actinium and protactinium occur in nature as daughters of thorium and uranium, while small amounts of neptunium and plutonium are present as a result of neutron-capture reactions of uranium. All other members of the series are man-made. Separation chemistry has been central to the isolation and purification of the actinides since their discovery. The formation of the transplutonium actinides was established only as a result of chemical-separation procedures developed specifically for that purpose. The continued application of separation science has resulted in the availability of weighable quantities of the actinides to fermium. Separation procedures are central to one-atom-at-a-time chemistry used to identify synthetic trans-actinide (superheavy) elements to element 107 and above (Report of a Workshop on Transactinium Science 1990). 

A good, if at some points rather dated, survey is Complexes of the Rare Earths, S. P. Sinha, Pergamon, Oxford, 1966. An excellent recent account of the chemistry of the lanthanides and actinides is to be found in Lanthanides and Actinides, S. Cotton, Macmillan, London, 1991. An excellent series of brief review articles is to be found in Radiochim. Acta (1993) 61. Three examples are Overview of the Actinide and Lanthanide (the f) Elements by G. T. Seaborg (p 115), Systematics of Lanthanide Coordination by E. N. Rizkalla (p 118) and, for those who are interested in learning how to study the chemistry of a couple of dozen atoms, Atom-at-a-Time Chemistry by D. C. Hoffman (P 123). 

Fundamental and Experimental Aspects of Single Atom-at-a-Time Chemistry... 

Studies of the chemical properties of the heaviest actinides (Z > 101) and aU of the transactinides (Z > 103), including superheavy elements (SHEs), depend on the use of atom-at-a-time chemistry. They cannot be produced by simple neutron... 

The discovery and identification of element 101 (mendelevium, Md) was a landmark experiment in many ways [ 1 ]. It was the first new transuranium element to be produced and identified on the basis of one-atom-at-a-time chemistry and it is also the heaviest element (to date) to be chemically identified by direct chemical separation of the element itself. All of the higher Z elements have been first identified by physical/nuclear techniques prior to study of their chemical properties. In fact, one of the criteria for chemical studies is that an isotope with known properties be used for positive identification of the element being studied. Due to relativistic effects [1] chemical properties cannot be reliably predicted and a meaningful study of chemical properties cannot be conducted with both unknown chemistry and unknown, non-specific nuclear decay properties ... 

The chemical information deduced from these experiments, illustrative of one-atom-at-a-time chemistry, appears reliable. Intuitively, the numerous repetitive experiments give statistically significant results see Liquid-Phase Chemistry of SuperheavyElements and Gas-Phase Chemistry of Superheavy Elements for a comprehensive report of SHE chemistry experiments in the liquid-phase and gas-phase, respectively. The next paragraph will provide proof that kinetics and thermodynamics are valid at the atom scale for most chemical reactions performed with SHEs. 

However, reactions between Ei and E2 can only be observed if the half-life of El is compatible with the time characteristics of the reaction under study. For instance, the time needed to achieve the equilibrium must be shorter than the lifetime of the involved radionuclide. Studies of the chemical properties of SHEs give rise not only to the concept of single-atom chemistry but also to one-atom-at-a-time chemistry. For that purpose, chemical processes with high reaction rates are required. 

The feasibility of Kj determinations in the context of one-atom-at-a-time chemistry is very promising and the collection of Kj values will allow establishment of reliable variations of the chemical properties (complexation, hydrolysis) of elements within a group, for comparison with theoretical predictions, and, perhaps, for determination of thermodynamic constants. Moreover, other information can be derived from chromatography experiments. The mathematical treatment of elution curves can be carried out with various models, especially Glueckauf s, which offers the advantages of using simple equations and takes into account the possible dissymmetry of elution bands [31, 32]. The parameters included in Glueckauf s equations allow the determination of the distribution... 

HoSman, D.C. Atom-at-a-time chemistry. Radiochim. Acta 61, 123-128 (1993)... 

Due to the extremely low production rates of transactinides in nuclear fusion reactions, all chemical characterizations are carried out at the single atom level (see chapter Fundamental and Experimental Aspects of Single Atom-at-a-Time Chemistry ). The chemical reaction products are characterized on the basis of their behavior in the separation process or, to be exact, in the gas-phase-adsorption chromatographic process (see Part I of this chapter). In this process the formation probability of defined stable chemical states of transactinides and the subsequent interaction of the formed species with a solid state surface are studied. 

Chapter 4 discusses fundamental questions of the validity of chemical information obtained one atom-at-a-time. While stiU presenting concepts of statistical thermodynamics and fluctuation theory, and discussing limitations of atom-at-a-time chemistry, the revised version of this chapter includes a discussion of atom-at-a-time chemistry in more general terms. 

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