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Pure Appl. Chem., Vol. 63, No. 6, pp. 879-886, 1991.

Criteria that must be satisfied for the
Discovery of a New Chemical Element to be Recognized







I.1. The idea that a few elementary substances could, combined in various ways, build up the whole of matter goes back to the Greek philosophers of the sixth century B.C. The earliest form of the atomic "hypothesis", more or less as we know it today, goes back almost as far, to Leucippus in the fifth century B.C. later elaborated by Democritus. The dead hand of Aristotle relaxed its grip only in the 17th century: atomic science began to emerge from the atomic philosophy of Democritus with Robert Boyle's (1661) recognition of chemical elements as "certain primitive and simple bodies not being made of any other bodies or of one another". In the late 18th century Antoine-Laurent Lavoisier, defining a chemical element as something that could be decomposed by chemical means into simpler substances, recognized about thirty of what we now know to be elements in the modern sense but also included a few particularly stable chemical compounds that resisted attempts to break down. John Dalton, in 1808, introduced the idea of atomic weight as a useful characterization of a chemical element, and chemists began to seek relationships between elements on the basis of such weights. The great step was taken by Dmitri Mendeleev in 1869 who recognized that the chemical properties of the elements (of which he knew about 65) were periodic in their atomic weights; this permitted him to construct his periodic table and to make the dramatic prediction of the existence of then-undiscovered elements, notably eka-boron (scandium), eka-aluminium (gallium), and eka-silicon (germanium). With the discovery of isotopes by J.J. Thomson in 1912 following their inference by Frederik Soddy in 1910, and with the discovery of characteristic X-rays by H.G.J. Moseley in 1913, atomic charge or atomic number placed atomic weight as the chief determining character of an element.

The centuries-old history of the definition and discovery of chemical elements has a deep scientific and general fascination. This is because the problem is of an essentially finite scope: there can only be a limited number of species of atomic nuclei containing different numbers of protons that can be imagined to have an existence, though perhaps only fleeting, in the chemical sense. But although the problem is of finite scope, we do not know what the scope is: we do not as yet know how many elements await discovery before the disruptive Coulomb force finally overcomes the nuclear attraction. In this sense, the problem is open although of finite scope, unlike the number of continents upon the surface of the earth where we know with certainty that none still awaits discovery. These considerations give to the discovery of new elements an importance, an allure and a romance that does not attach to the discovery of, say, a new comet or a new beetle where many more such discoveries are to be anticipated in the future. This, together with, of course, the insight that they give into the details of the construction of Nature's most complex nuclear edifices and the laws that govern their construction, explains the great investment of material and, most particularly, human resources into the discovery of new elements. Lives are committed over decades to this enterprise, and this is not surprising. Nor is it then surprising that, although from the point of view of science itself (except that of the "science of history") and the associated advance of human understanding it does not matter who makes the discovery, immense importance is attached, personally, institutionally and nationally, by those engaged in the enterprise, to the public recognition of their discoveries. Not less surprising is that differences of opinion have arisen, in certain cases, as to the relative importance of the various contributions, by various research groups, at various times, that might have been made to the recognition of such new elements.

Specifically, such differences of opinion have arisen concerning priority in the discovery of elements beyond Z=101. In part, these difficulties have been caused by a change in the experimental methods needed to produce these elements. The reported discoveries have been made using complete-fusion/neutron-evaporation reactions. In them, overwhelming competition with prompt fission causes low yields which, combined with the often quite short half-lives, made experimental investigation difficult. Also, backgrounds with properties similar to those expected for transfermium elements but in reality due to lower-Z products of other type (transfer-) reactions, or to reactions on impurities (especially in actinide targets), have sometimes caused confusion.

I.2. It has long been felt that the scientific community should be able to resolve differences of opinion such as those which reference has just been made by appropriate objective study of the evidence. In 1974 IUPAC in collaboration with IUPAP appointed a group of experts not themselves directly involved in the controversies, 3 from the USA, 3 from the USSR and 3 (including the chairman) from other countries, specifically ".. to consider the claims of priority of discovery of elements 104 and 105 and to urge the laboratories at Berkeley (USA) and Dubna (USSR) to exchange representatives and in their presence to repeat the experiments regarding these elements." This committee never completed its work, nor issued a report nor, indeed, met as a group, though there was some correspondence between members.

At its meeting in September 1985 the Executive Council of IUPAP, faced with renewed calls for the solution of the continuing problem and in view of the evident lapsing of the earlier initiative, decided to suggest to IUPAC the establishment of a new joint group, the Transfermium Working Group (TWG), to examine the issues ab initio. It was decided that the members should not be drawn from the countries of the major laboratories concerned with research into the heaviest elements which in addition to the USA and the USSR now included West Germany through GSI in Darmstadt.

I.3. The Terms of Reference of the TWG as suggested by its Chairman were approved by the Executive Council of IUPAP in September 1986 and by the IUPAC Bureau in October 1986; they include that the work be divided into two phases:

Phase (i) The establishment of criteria that must be satisfied for the discovery of a new chemical element to be recognized;
Phase (ii) The application of these criteria in practice.

It was also agreed by IUPAP and by IUPAC that: "(TWG in Phase (i)) should to some degree work interactively with the major experimental laboratories concerned so as to move to criteria that will command general assent."

I.4. IUPAP membership of the TWG was determined by IUPAP in September 1987 and the IUPAC membership by IUPAC at that same time:


D.H. Wilkinson (UK) Chairman
R.C Barber (Canada)
A. Hrynkiewicz (Poland)
M. Lefort (France)
M. Sakai (Japan)
I. Ulehla (Czechoslovakia)
A.H. Wapstra (Netherlands)


N.N. Greenwood (UK)
Y.P. Jeannin (France)

The TWG appointed Messrs Wapstra and Ulehla as its joint secretaries.

I.5. The TWG has held the following meetings, of which the first and last were "private", with the remainder in the laboratories of chief concern:

3-5 February 1988 Nonant (France)
12-17 December 1988 Darmstadt (FRG)
19-23 June 1989 Berkeley (USA)
12-16 February 1990 Dubna (USSR)
16-20 April 1990 Prague (Czechoslovakia)

At the meetings in the laboratories, the TWG divided its time roughly equally between "private" meetings and sessions with the scientists of the laboratories. The TWG is most grateful for the warm and open discussions that it has enjoyed in all three laboratories.

I.6. Early in its work, the TWG realized that it was not practicable to effect a clean separation between the establishment of criteria and their application in the sense that the validity of criteria, and the pitfalls that might be encountered in their application, can be assessed only through discussion of their potential impact in real examples. The TWG has, as far as possible, restricted its examination in Phase (i) of such examples to cases that are not contentious in respect of priority claims but only by such "shadow" application of the evolving criteria has it been possible to establish a set of criteria that is indeed applicable in practice. These shadow exercises, not infrequently in respect of cases in which reported assignments to certain nuclides were later found to be incorrect, have proved to be most informative.

A benefit of this practical evolutionary approach to the definition of criteria is that the TWG is now fully ready to proceed to Phase (ii) of operations, the adjudicatory phase, should the present report on Phase (i) be accepted by IUPAP and IUPAC, who have agreed that the TWG as presently constituted should remain unchanged for Phase (ii).

I.7. The TWG has also come to the conclusion that it is not feasible to specify criteria, or combinations of criteria, that, in the words of its Terms of Reference "must be satisfied ..." in order to achieve recognition of the existence of a new chemical element and that would cover all cases. Very few properties indeed, of which perhaps the only uncontentious example is the characteristic X-ray spectrum, unambiguously determined, are suficient themselves to establish the existence of a new element. For the rest, identification must rely upon combinations of properties that will vary from case to case and that cannot usefully be exhaustively codified as a set of criteria *.

To adopt any such codification would be to force research into a strait jacket inimical to the spirit of free enquiry. The TWG has therefore discussed, and here presents, those criteria (section II) and properties (sections III and IV) that have been used in the past and that are seen as being of relevance for the future, and gives some indication of the store that it sets by them, but recognizes that their relative importance will vary from case to case depending upon the circumstances in which they are displayed and the manner in which they are combined

To this degree, therefore, the TWG departed from the letter of its Terms of Reference and would draw attention to the resultant disjunction between the title of this Report, taken from those Terms, and its contents; it holds strongly, however, that the position it presents in this Report is the correct one.

I.8. The TWG has had to adopt an historical perspective in establishing its criteria and testing them through the shadow exercises but of course, the application of the criteria lies largely in the future. However, it is evident that, historically, new elements were proposed, and accepted, on the basis of evidence that would not meet the criteria of today, even prior to the codification upon which the TWG has been engaged. In terms of published scientific evidence and also in terms of the public presentation of that evidence, there often appear what we can only describe as significant inadequacies. The standards of the times have markedly tightened, not least because of the development of new experimental technologies and, particularly, of the computer. However, this presents us with a problem which must be recognized at this stage although its impact will be felt only in Phase (ii), namely that some of the contentious cases, still to be resolved, now lie in the fairly remote past at a time when standards were different from those of today. We must be constructively sensitive to this when we move to Phase (ii). It would not be fair, or indeed possible, to apply to yesterday the criteria of today without regard for the circumstances of the times.

I.9. The TWG has not been charged with, and will not express opinions about, any matter to do with the naming of the new elements, either in Phase (i) or in Phase (ii). The following information may be useful:

The body primarily concerned with recommending names for the new elements is the IUPAC Commission II.2 on the Nomenclature of Inorganic Chemistry. This Commission does not "decide" on the adoption of names, but only publishes recommendations for international use. The 1990 version of its "Red Book" ("Nomenclature of Inorganic Chemistry". Blackwell Scientific Publications, Oxford) states in chapter I.3:

"The names approved by IUPAC are based on considerations of practicality and prevailing usage. It is emphasized that the IUPAC selection carries no implication regarding priority of discovery."

Systematic names have been proposed by IUPAC for all elements with Z=100-999. These names were only intended for use in the period when no "official" names were yet available. However, they have not met with favour among nuclear chemists nor among physicists. They will not be discussed in either phase of the present work.

Although the TWG is not concerned with names it feels that, in due course, following its Phase (ii) report, it would be felicitous if the laboratories concerned in the work leading to the establishment of a new element were to present to IUPAC an agreed joint suggestion as to the name.

I.10. Another matter involving IUPAC custom and practice is of concern here: the TWG will follow the IUPAC Red Book definitions for elements and isotopes and their atomic number Z, atomic mass number A and atomic mass M (in atomic mass units). In cases where the difference is important or instructive, the term "isotope" (which strictly speaking refers to two or more atomic species having the same atomic number Z but different atomic masses) will be replaced by "nuclide" (for which no such restriction applies.) We also refer to the Red Book for the notation for nuclear reactions and for the definition of their cross sections.

I.11. It has become very clear to the TWG, particularly in its review of historical cases referred to in I.6 above, that the situation in respect of the discovery a new element is by no means always black-and-white in the sense that it may be unequivocally asserted that a new element was discovered, with the required certainty, by a certain group, using a certain method on a certain date. Sometimes this is the case, and this is what is popularly thought of as "discovery". Perhaps more often , however, the situation is one in which data accumulated over a period of time, perhaps of years, perhaps in two or more laboratories, gradually bring the scientific community to the conviction that indeed the existence of a new element has been established. However, different individuals or different groups may take different views as to the stage in the accumulation of evidence at which conviction is reached and may take different views as to the existence or otherwise of crucial step leading to that conviction and as to which those crucial steps were. Such differences can be perfectly legitimate scientifically, in that they may depend upon, for example, differing views as to the reliability of the inference that might be drawn from certain types of evidence, while not disputing the reliability of the evidence itself. So, although scientific community may reach consensus as to the existence of a new element, the reaching of that consensus is not necessarily a unique event and different views may, in all scientific honesty, be taken as to the steps by which it was reached.

It will be the business of the TWG, in Phase (ii), to analyze these cases in detail and to attempt, where the situation is indeed not black-and-white, to trace, with the aid of criteria now to be discussed, a kind of discovery profile and to delineate the steps by which certainty in "discovery" was reached and, if possible, to indicate the relative importance of those steps. In other words, it is the TWG's conviction that there may be cases in which it would be unjust to assign an absolute priority in the "discovery" of a new element but where the credit should be appropriately apportioned.

The TWG is sensitive to the fact that it may be thought by some that such an apportionment of credit would be shirking the issue. This, however, is not the case and such an opinion could be held only by those disregarding the high complexity of the researches involved and the frequent lack of total specificity in their application. Demand for absolute priority assignments in all cases related to the new elements would imply an attachment to outmoded concepts of the nature of discovery.

I.12. The TWG realizes, consonant with its views expressed in I.11, that certain things relating to discovery will have to be made matters of individual definition in the they stand outside scientific criteria as such. An example of this might be Paper I that presents evidence relating to a possible new element but which is not adequate of itself to establish the existence of the new element without the evidence published later in Paper 11, perhaps from a different laboratory, that, together with Paper I, carries certainty but which is also not sufficient of itself. It is now certain the Paper I "saw" the new element but could not prove it at the time. Where does the priority lie? Is it with Paper I or is it shared between Paper I and Paper II? And if, between Paper I and Paper II, there was published a Paper III that was complete in itself and carried conviction, does the credit lie wholly with Paper III even though, after the publication of Paper II, it is evident that Paper I saw the new element first?

We draw attention to such problems not to propose universal and rigid resolutions such as might lead to the assignment of absolute priorities but rather to support our concept of the discovery profile which we feel will lead to a more equitable appreciation of the range of contributions that might have been made. An absolute priority would often equate to an absolute injustice.

A similar situation in which the discovery profile would offer a fairer assessment would be one in which an early paper could not, at the time, carry conviction but which was later realized to have reported correctly signals from the new element in question, the existence of which was definitely established by subsequent work following up the lead of the early paper. Although it would clearly be wrong to assign absolute priority to that early paper, it would, in our view, be appropriate to recognize its seminal importance.

The discovery profile will also accommodate cases in which two groups correctly report a new element within a brief time interval of each other and in evident independence. In the TWG's view it would be absurd and unjust to accord an absolute priority to the group that, in such circumstances, simply happened to submit first. The discovery profile will enable people who wish to attach importance to such matters to do so.

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We distinguish between properties and criteria. Properties are the objective chemical or physical attributes of atoms and of nuclei or of processes through which they and their behaviour are categorized. Criteria are the conditions that must be met for those properties to be admitted as diagnostic in respect of the character (e.g. the Z-value) of the bodies concerned. (Back to text)

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