Darleane C. Hoffman is scheduled to present the Priestley Medal Address on March 28 during the award ceremony at the American Chemical Society's 219th national meeting in San Francisco. Hoffman is a chemistry professor in the graduate school of the University of California, Berkeley, and faculty senior scientist and co-group leader of the Heavy Element Nuclear & Radiochemistry Group at Lawrence Berkeley National Laboratory. She is being honored for her work investigating chemical properties of the heaviest elements on a one-atom-at-a-time basis that has been important in elucidating the placement of those elements in the periodic table. Hoffman also is being honored for her contribution to education in the field of nuclear and radiochemistry. Following is the text of her address.

I have chosen to call this article "The New Millennium" to signify change and new beginnings. In the interests of brevity, I must limit my discussion to two topics that are near and dear to my heart and with which I can claim some familiarity--the recent spectacular advances in heavy-element nuclear and radiochemistry and changes in the status of women in chemistry over the past 50 years.

1999: A Banner Year for Heavy-Element Nuclear and Radiochemistry

Historically, the production of new elements and new heavy-element isotopes and studies of the chemical and nuclear properties of the radioactive elements have been primary goals of the University of California, Berkeley/Lawrence Berkeley National Laboratory (LBNL) Heavy Element Nuclear & Radiochemistry Group. Under the direction of the late Glenn Seaborg and then Albert Ghiorso, the transuranium elements were extended through element 106, later to be named seaborgium.

The sometimes unexpected and unique chemical properties of these elements were also investigated at Berkeley and elsewhere. However, after discovery of element 106 in 1974, the focus for the discovery of new elements shifted to Gesellschaft für Schwerionenforschung (GSI) in Germany, where the new in-flight separator for heavy-ion reaction products had been built. It was used to identify the next six elements: bohrium (107) through meitnerium (109) between 1981 and 1984, and elements 110 through 112 between 1994 and 1996 (Figure 1).

As the pace of discovery of new elements slowed, interest in studies of their chemical properties declined precipitously from the 1970s until the mid-1980s. Then a renaissance of interest in the chemistry of lawrencium (103) and the transactinides was sparked by predictions that relativistic effects might cause changes in their electronic structures, which would result in deviations in their chemical properties from those predicted by simple extrapolation of known trends in the periodic table.

[LBNL photo]

The long-sought superheavy elements

The existence of superheavy elements (SHEs) was predicted as early as 1955 by John Wheeler and discussed by Gertrude Scharff-Goldhaber in 1957. But the big wave of interest and serious searches for SHEs did not begin until the 1960s, spurred by theoretical predictions of an "island of superheavy elements" well beyond uranium.

This island was calculated to be in the region of the spherical nuclear shells (or "magic numbers") at atomic numbers 114 or 126 and neutron number 184, as shown in the topographical representation in Figure 2. Predictions of half-lives as long as a billion years prompted searches in natural ores for element 114 as eka-lead and even for element 126, now expected to be part of a "superactinide" series which begins with element 122 and ends with element 153 after filling of the 5g and 6f electronic subshells (Figure 3).

First attempts to produce SHEs at accelerators were initiated at Berkeley in 1968, but with no success. Although there were many reports of evidence for SHEs both in nature and at accelerators, no confirmed results had yet been obtained by 1976.

During the August 1976 ACS national meeting in San Francisco, Glenn Seaborg gave his ACS presidential address entitled "Chemistry--Key to our Progress" to mark the 100th Anniversary of ACS. He not only pointed out the many advances in fundamental knowledge and the contributions and applications of chemistry to society over the previous 100 years, but made numerous predictions about future developments.

One of these that particularly caught my attention was, "Nuclear chemists will be involved in the synthesis of additional chemical elements, hopefully in the 'island of stability.' " Although many extremely sensitive physical and chemical methods were developed and used in the search, no confirmed evidence for the existence of SHEs remained by the end of the 1980s, and the searches were abandoned.

But the production of elements 110 to 112 in 1994-96 at GSI created renewed optimism because they decayed predominantly by emission rather than spontaneous fission, contrary to earlier predictions. The subsequent attempts to produce element 113 were unsuccessful, and the GSI group decided to delay further attempts while they improved their separator.

At about the same time, our group embarked on a project to build the Berkeley Gas-filled Separator (BGS) based on a new design by Ghiorso to achieve greater sensitivity than existing separators and allow us to reenter the quest for still heavier elements. Ken Gregorich, one of Seaborg's last Ph.D. students and my first postdoctoral fellow after coming to Berkeley in 1984, was appointed to head the project, and Victor Ninov from GSI was brought to help with the project.

Figure 1

By the fall of 1998, BGS was ready for testing, and the first experiments were performed in December 1998. Then the excitement began in January 1999 when we learned that a collaborative group from the Flerov Laboratory of Nuclear Reactions, Dubna, Russia, and Lawrence Livermore National Laboratory had found evidence for a single event attributable to element 114 in data obtained from some 40 days of running time with their gas-filled separator at the Dubna cyclotron during November and December 1998. They believed it was due to element 114 with a mass of 289 formed in the "hot" fusion reaction of ⁴⁸Ca projectiles with ²⁴⁴Pu targets in which three neutrons were emitted from the compound nucleus.

From their measurements of the time intervals between three successive decays followed by spontaneous fission, they estimated relatively long half-lives of about 20 seconds, 10, 1, and 11 minutes, respectively, for the observed element 114 and its element 112, 110, and 108 daughters. We debated whether to try to repeat their experiment but, instead, based on the predictions and suggestions of Robert Smolaczuk, a Fulbright Scholar visiting with our group from the Soltan Institute for Nuclear Studies, Warsaw, we decided to try the "cold" fusion reaction of ²⁰⁸Pb targets with ⁸⁶Kr projectiles.

These experiments were undertaken at the LBNL 88-inch cyclotron in April and May 1999. BGS was used to separate three events attributed to element 118 with a mass of 293 based on decay by a unique series of high-energy particles to the new elements 116 and 114, and to previously unknown neutron-rich isotopes of elements 112, 110, 108, and 106. The three observed -decay chains from element 118 are shown in Figure 4. The decay energies and estimated half-lives of 0.12 millisecond, 0.60 millisecond, 0.58 millisecond, 0.89 millisecond, 3.0 milliseconds, and 1.2 seconds for the chain members were in remarkable agreement with Smolaczuk's predictions. In two of the chains there is evidence that the half-life of ²⁶⁹Sg might be as long as 20 seconds.

In July 1999, a multinational group, also working at Dubna, published evidence for two different decay chains of element 114 consisting of decay followed by spontaneous fission produced in the reaction of ⁴⁸Ca projectiles with ²⁴²Pu targets. They attributed these events to the decay of element 114 with mass 287 and a half-life of about 5 seconds based on observation of a spontaneous fission half-life of about 4.5 minutes for the daughter. They believed this to be the same as the 1.4-minute spontaneous fission activity that they produced previously in the reaction of ⁴⁸Ca with ²³⁸U targets and attributed to element 112 with mass 283. But identification based only on the similarity in half-life of this unconfirmed spontaneous fission activity is rather uncertain.

So suddenly and rather unexpectedly, the new superheavy elements 118, 116, and 114 have burst upon the scene. In addition, there is evidence for three isotopes of element 114, three new isotopes of element 112, two new isotopes of elements 110 and hassium (108), and one new isotope of seaborgium (106).

If we assume that all these reports are confirmed, then the number of known nuclides beyond element 105 has increased from 23 to 39 within a single year, and most of them are longer lived than previously known (Figure 5). This certainly bodes well for the extension of chemical studies to still heavier elements. The "gaps" shown in the time line for the discovery of the transuranium elements (Figure 1) emphasize that the discoveries come in spurts after the development of new concepts, new production reactions, new techniques, or new instrumentation needed for the production, separation, and identification of each successive "group" of new elements.

Figure 2

What are the chances for filling in the missing elements 119, 117, 115, and 113? Smolaczuk predicts that element 119 with mass 294 will decay via a high-energy -decay sequence that produces the new elements 117, 115, and 113, and finally ends with the known 3.6-hour isotope ²⁶²Lr. Obvious choices for element 119 production are the use of lead targets with ⁸⁷Rb projectiles or bismuth targets with ⁸⁶Kr. The production rates for these reactions are expected to be almost as good as the one used to make element 118. Experiments to determine the optimum bombarding energy for producing element 118 will help us design experiments to maximize the production rates for elements 119 and 120 and new longer lived isotopes that might be "stockpiled" for chemical studies.

Figure 3

Chemistry of the heaviest elements

Now let us return to the chemistry of the heaviest elements. By 1974, the elements through seaborgium (106) were known, and by the early 1970s investigations of chemical properties had been extended through rutherfordium (104). These studies showed that the actinide series was completed at lawrencium (103) as predicted by the actinide concept, and that rutherfordium, unlike the actinides, showed properties similar to hafnium and zirconium. Therefore, it should be placed under them in the periodic table (Figure 3) as the first member of a new, heavier 6d transition series.

By 1984, the elements through meitnerium (109) had been discovered, but studies of chemical properties of the heaviest actinides and transactinides were certainly lagging behind. But as mentioned earlier, chemical theoreticians predicted that due to relativistic effects large deviations from properties predicted by simple extrapolation of periodic table trends might occur, and in the mid-1980s our group reinitiated studies of the chemical properties of the heaviest elements.

One of the reasons the chemical studies had languished is that they require special facilities and capabilities, which exist at only a few laboratories in the world, and they present unique challenges to the chemist. Because of the short half-lives and low production yields for these elements, they must be studied on-line at the accelerator where they are produced. They must be detected by measuring the radioactive decay of a single atom (one-atom-at-a-time chemistry), and the results of many separate experiments must be combined to get statistically significant results. Furthermore, the half-lives of the species being investigated must be long enough to allow chemical equilibrium to be achieved, thus limiting the type of chemistry that can be performed and the half-life of the isotopes that can be used.

Our current techniques typically require half-lives of at least a second. Thus, chemical studies must await discovery of sufficiently long-lived isotopes and knowledge of their nuclear decay so that the element being studied can be positively identified. After all, we cannot study unknown chemistry unless the isotope being studied can be positively identified as belonging to the element whose properties we are trying to investigate.

Calculations of relativistic effects in the mid-1980s suggested that they might so strongly stabilize the 7s electrons in element 105 that only three rather than five electrons could be easily removed from its valence shell, and then its most stable state would be trivalent like the actinides. On the other hand, simple extrapolation of periodic table trends indicated that it should be placed after rutherfordium (104) and under tantalum as the next member of the 6d transition series and the heaviest member of group 5. Therefore, it would be expected to resemble tantalum and exhibit a most stable pentavalent oxidation state.

No investigations of the solution chemistry of element 105 had yet been performed, so we decided to try to design an experiment to choose between these conflicting predictions. Günter Herrmann, of the University of Mainz, Germany, who was visiting with us at the time, suggested a simple chemical separation to distinguish between these oxidation states, based on the tendency of the 5+ state to sorb on glass surfaces whereas the 3+ ions did not. It took about 50 seconds, and more than 800 of these separations were performed manually using a 35-second isotope of element 105. The results showed that it sorbed on glass as did tantalum and that it should be placed in group 5 under tantalum in the periodic table.

But surprisingly, in some chemical systems, element 105 behaved more like its lighter group 5 element niobium than like tantalum, suggesting a reversal in trend in going down group 5 of the periodic table. And sometimes it was even more like protactinium, a pentavalent actinide.

Figure 4

Note: Times are intervals between successive decays, from which   half-lives can be derived. Source: Phys. Rev. Lett., 83, 1104 (1999)

Similarly, rutherfordium (104) behaved more like the lighter group 4 element zirconium than like the heavier homolog hafnium in gas-phase studies of halide volatilities, while sometimes in liquid-liquid extractions it resembled the early actinides plutonium (4+) or thorium (4+).

These unexpected results stimulated continuing interest in trying to understand the complex chemical properties of the heaviest elements and led to international collaborations of scientists who conducted experiments at both LBNL and GSI. Computer-controlled, automated on-line systems were developed for both aqueous and gas-phase chemistry and greatly facilitated the hundreds of repetitive separations required to obtain more detailed information for comparison with predictions from the newly developed, relativistic molecular orbital calculations.

Because relativistic effects on the electronic orbitals are expected to increase as the square of the atomic number, we have constantly endeavored to extend our chemical studies to the heaviest possible elements. With the report in 1995 by a Dubna/LLNL group of the production of the longer lived isotopes of seaborgium with half-lives of 10 to 30 seconds, our collaboration under the leadership of Matthias Schädel from GSI began plans to study both aqueous and gas-phase properties of seaborgium.

These have been performed over a period of several years and have shown that the element's properties are generally similar to those of molybdenum and tungsten, and it seems to have found its proper place as the heaviest member of group 6 of the periodic table. But again, there are similarities and differences, which need further investigation to determine if they can be correlated with calculations of relativistic effects.

Bohrium (element 107) finds a place in the table

I use this title here exactly as it appeared in a full-page news article, complete with a schematic diagram of the experiment, in the January/February 2000 issue of the CERN Courier (International Journal of High-Energy Physics). We radiochemists consider it somewhat incredible to receive such recognition by the high-energy physics community.

The bohrium story began early last year when our group decided that in order to study its chemistry we must try to produce a longer lived isotope than the currently known 0.4-second ²⁶⁴Bh. The new, heavier isotopes of bohrium of mass 266 or 267 were predicted by Adam Sobiczewski, Zygmunt Patyk, and colleagues at the Soltan Institute to have half-lives on the order of 10 seconds.

Figure 5

Relying on the strength of these predictions and a successful outcome to our search for these isotopes, our Swiss colleague Heinz Gäggeler at the University of Bern and the Paul Scherrer Institute (PSI) requested some 30 days of beam time for an international collaboration to perform the first chemical studies of bohrium using the on-line gas analyzer system at PSI's Phillips cyclotron. In March 1999, we irradiated radioactive²⁴⁹ Bk targets with ²²Ne projectiles at the LBNL 88-inch cyclotron to try to produce these new isotopes prior to the chemistry experiment. And indeed, in about 10 days of running time, we were able to identify five events attributable to ²⁶⁷Bh based on the time correlation between its decay and the decay of its known daughter, ²⁶³Ha. The half-life of ²⁶⁷Bh was determined to be about 20 seconds, long enough for the proposed gas-phase chemistry experiments.

An international group conducted these experiments at PSI in fall 1999 using the same reaction. Six atoms were detected and positively identified by their known -decay chains, which showed that bohrium formed an oxychloride similar to that of its lighter homologs technetium and rhenium and unlike the actinides or the neighboring transactinides. Therefore, bohrium has found its proper place in the periodic table as the heaviest member of group 7. But more detailed studies will be conducted later this year to further explore its properties and determine if there is a reversal of trends in going down group 7 as there is in groups 4, 5, and 6. These bohrium experiments again illustrate the synergism and even necessity of carrying out hand-in-hand studies of the nuclear and chemical properties of the heaviest elements.

Plans are now being made to extend our chemical studies to hassium (108), probably using the known 9-second²⁶⁹ Hs. Chemical separation based on the volatility of its tetroxide, which is expected to be similar to that of the group 8 element osmium, is being developed using BGS as a preseparator to remove other possible volatile contaminants.

The new element 118 and 114 landing sites and the new heavy-element isotopes discovered since 1978 are shown on the topological representation of the heavy-element region and in the originally predicted island of spherical SHEs in Figure 2. New isotopes with measurable half-lives can be produced all along the path to the former island of stability, so it is no longer a remote island. Many isotopes are predicted to live long enough for studies of their chemical properties. The -stable nuclide²⁹² 110 is expected to undergo decay with a half-life of about 50 years. But new and imaginative production reactions, new techniques for utilizing higher beam currents, and methods for accumulating reaction products for future off-line studies must be devised if we are to fully explore the extent of this exciting new landscape of diverse chemical and nuclear properties that is opening before us.

Status of Women in Chemistry--50 Years Later "You've Come a Long Way, Baby." BUT . . .

My first inclination was to call this part of my talk simply, "We've Come a Long Way, Baby," paraphrasing the slogan made famous in the cigarette ads of some years back, which showed a series of rather cynical cartoons graphically highlighting women's newfound independence and freedom since the beginning of the 20th century. Of course, by inference the ads included the freedom to smoke as one of these. Then as I was preparing this Priestley Medal Address, I read with great interest the ACS Comment in the Jan. 31 issue of C&EN by Frankie K. Wood-Black, chair of the ACS Women Chemists Committee.

It was entitled "A picture of the future." She projected a future Priestley Medal Address in which the audience is "treated to a historical look at how the workforce and workplace have changed over the medalist's career. . . . The medalist remembers that these groups [women and minorities] held neither key positions in ACS--such as editors of ACS journals--nor full professorships or upper management positions at key chemical companies. The medalist recalls how it used to make the news when a member of one of these groups was appointed to a board of directors or was named chief executive officer of a large chemical firm."

I can fulfill the first part of her prediction, but unfortunately, not the last in which the medalist says that during her career the changes were so extensive that we were no longer concerned about the demographics of awards to women and minorities and how many hold which positions. So I decided to add the "BUT . . ." because even though the changes since I received my B.S. and Ph.D. degrees in chemistry in 1948 and 1951 have been so radical that they are almost unbelievable to today's young women, we certainly have not progressed to the point that Frankie Wood-Black projects in her vision for the future. We have, indeed, come a long way, BUT . . . we still have a long way to go.

First, let me point out some of the changes for the better that have occurred during my lifetime. I went to college during World War II, which ended in 1945 while I was still an undergraduate. During the war, women were desperately needed and actively recruited and paid very well to fill positions traditionally occupied by men in the U.S. When the war ended, it was not clear whether women would continue to hold these positions even though they had performed exceedingly well.

Prior to World War II, if a woman teacher in the public schools married, she was expected to resign immediately. If she perchance became a widow, then she might be rehired. I observed that the only female professors I had in college were spinsters--that is, unmarried older women. And they held appointments in home economics, regardless of their professional training.

This was even true of my inspiring freshman chemistry professor, Nellie Naylor, the person responsible for my choice of a career in chemistry rather than applied art. But when I made my decision to change to chemistry, I decided I wanted to follow the example of Marie Curie, whom I had read about in elementary school. She had married and had children and did not fit the mold of the spinster woman professor. Because of my observations, I steadfastly refused to get a teaching certificate as suggested by my school superintendent father, vowed never to teach, went on to graduate school, married, and then took a position at Oak Ridge National Laboratory while my husband, Marvin, remained in Iowa to finish his Ph.D. You can imagine the dire predictions about our future together that this behavior caused.

So, in some sense, I very early encountered many of the same problems that women still face today, as well as a few others (some merely inconveniences, others major) that have now gone away.

For example, as a married woman, even though I was the one with a career position, it was difficult to get a credit card in my own name. When my husband subsequently took a position at Los Alamos National Laboratory, I followed him there. When I tried to inquire about the position we believed I had been promised, I was told categorically by the personnel department, "We don't hire women in that division." (This later turned out not to be the case--the scientists were more reasonable than the personnel people and I was hired and stayed for 30 years.)

Originally, we regarded our positions as more or less temporary, but found the possibilities for obtaining two suitable positions at another location essentially impossible. Nepotism rules of universities and other institutions often prevented couples from both obtaining positions at the same place, although in general the national laboratories were more open-minded about this.

Then in the late 1950s and the 1960s there was an emotional backlash against women who worked outside the home; they were blamed for most of the ills of the time. This extended to the public school system where teachers asked children to have their fathers come to tell the class what they did but never even considered asking their mothers. Few men would admit to helping with household chores or child care. Young women in high school were often discouraged from entering the physical sciences even though they had excelled in their previous mathematics and science courses. All these things have changed dramatically, and many of these issues are no longer even items for discussion. We have, indeed, made tremendous strides.

BUT . . . our gains in some areas are rather spotty and seem to be the exception rather than the rule. In 1983, I was the first--and so far only--woman to receive the ACS Award in Nuclear Chemistry. This was also one of the first times that one of the scientific awards of the society had gone to a woman. That year I used the title, "You've Come a Long Way, Baby: Thirty Years as a Woman Scientist in a National Laboratory," for a talk I gave at the Women Chemists Luncheon.

At the time, I was division leader of the Isotope & Nuclear Chemistry Division (INC) at Los Alamos. I proudly pointed out that, as of June 1983, 27% of our INC employees were female, including 23% of the professional degrees versus the lab average of 17%, 50% of the chemical technicians versus the lab average of 27%, and 10% of the managers versus the lab average of 8%. Interestingly, 60% of our summer graduate students were women, but women constituted less than 10% of our consultants and visiting staff members, most of whom were older men. At the time, I assumed this meant that in the near future the percentage would increase rapidly as these young women progressed in their careers.

BUT . . . at the same time I was boasting about the large number of women in my division and presumably other national laboratories, I learned that in the Chemistry Division at Argonne National Laboratory there was only one female Ph.D. staff member, Marion Thurnauer, and the second one was not recruited until eight years after Thurnauer had been hired. Marion became actively involved in women-in-science programs and is now the Chemistry Division director there and continues to promote science and technology careers for women and minorities. Having gone from being the only woman to communicating with a network of other women, she emphasizes the importance of having a critical mass.

Women now have an equal opportunity to become scientists, especially chemists. Nationwide, the proportion of B.S. and Ph.D. chemistry degrees granted to women is about 50% and 33%, respectively. BUT . . . the percentage of tenured professorships held by women in the chemistry departments of major research universities remains very low.

When I went to Berkeley in 1984, there was only one woman, Judith Klinman, among the tenured faculty in chemistry. (And she will become our first female department chair as of July 1.) Both of us came in with tenure and since then three women have been appointed as assistant professors and have earned tenure. Recently, another woman joined our faculty in a tenure-track position. So now we are six women out of about 50 faculty members. Again, I believe this emphasizes that it really does take a critical mass to expand our numbers, but we must not become complacent and neglect our recruiting efforts.

When I gave the commencement address for our College of Chemistry at UC Berkeley in 1998, I was startled to find that I was the first woman to do so even though the percentage of women receiving B.S. and Ph.D. degrees in chemistry there was about 30%, and that the number enrolled in graduate school had increased from a relatively high 20% in the late 1980s to an average of about 33% in the 1990s (39% in 1999). Certainly we have succeeded in attracting women into careers in chemistry. In fact, women now constitute 20% of the ACS membership.

BUT . . . women are not similarly represented in tenured positions at major universities, in the attainment of other prestigious appointments or management positions, or in the receipt of major awards in ACS or similar professional societies.

Hoffman, her granddaughter Sarah, and her daughter Maureane Hoffman (a professor at Duke University Medical School) at the award ceremony for the 1997 National Medal of Science. [Photo by Christy Bowe/ImageCatcher News]

Why is the interest in academic careers among women graduate students declining? Our newly found opportunities over the past 50 years have brought new responsibilities and new kinds of problems that still remain to be solved--not just by women, but by men and women working together. We still have problems with two-career marriages: Whose career takes precedence in case two suitable positions can't be found near the same location, or who commutes?

We suffer from stress and overwork and try to be "superwomen." We wrestle with decisions about whether or not--and even when--to have children, and how to share responsibility for child care. Possibilities of sharing a single position or flex time have been suggested. Or do we turn our children over to nannies, child care facilities, trusted relatives, or possibly the "state"?

The concerns of young women in science about how to balance a scientific career with starting a family were eloquently discussed in a letter from Veronika Szalai recently published in C&EN ( Feb. 14, page 13 ) and have not changed all that much since my time. I chose to defer having children for several years in order to establish my career and then was very fortunate in finding a suitable nanny. And later my own mother came to live near us and help with the children. We still have not found global solutions to all these concerns.

Although I originally refused to consider teaching as a career, under present-day circumstances I have found it to be one of the most rewarding (and demanding) endeavors of my career. I also hasten to add that along with the freedom to work very long days we have more flexibility than less well educated women who may be constrained to boring jobs and strict eight or 10 hour days.

Now that our numbers have increased, women should take the initiative in proposing qualified women for awards as well as for the coveted appointments and management positions in both university and industrial settings. We must also enlist the aid of our male colleagues in these endeavors. In the past, they have taken the lead in proposing us; now we women must take a more active role in the process.

In closing, I would like to quote a few excerpts from Glenn Seaborg's 1976 ACS presidential address that are still applicable today: "Our success in chemistry, and science in general, over the past century, and especially the last few decades, has brought us to a high level of material affluence, but this success also has fostered many new problems for the world. It has given many people the notion that science should move us toward a utopian, problemless, riskless society. But this is a false notion. We live and always will live in a dynamic situation, amid problems whose solutions will breed other problems. . . . There always will be dangers, risks, and increasing responsibilities that will drive us toward a new level of excellence in all we do or try to achieve."

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