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Addressing the Group 3 Dilemma: A New Perspective

Boyang (Bobby) Zhang

Do lanthanum (La) and actinium (Ac) or lutetium (Lu) and lawrencium (Lr) belong to group 3 under scandium (Sc) and yttrium (Y), along with the d-block elements, in the periodic table?

Abstract

More than three decades after William Jensen’s article initiated the group 3 debate of the periodic table in 1982, the chemistry community has not yet reached a consensus towards whether lanthanum (La) and actinium (Ac) or lutetium (Lu) and lawrencium (Lr) should be placed in group 3 under scandium (Sc) and yttrium (Y). It is desirable to have a recommended version of the periodic table since multiple versions of periodic tables can be confusing for many people. Though there is specific evidence that validates the placement of lanthanum and actinium, and though it is important to consider chemical properties, such an approach is limited in scope, leading to conflicting conclusions. Recently, more conclusive arguments that promote the Lu, Lr arrangement can resolve this issue by approaching it from a global perspective, emphasizing that the periodic table should be organized strictly by overall principles.

The iconic periodic table created over two centuries ago has recently fallen into an awkward situation — it has been questioned whether it assigns every element to its rightful place. The elements in group 3 start with scandium (Sc) and yttrium (Y), but what are the next two elements? Periodic tables across textbooks, posters, and websites may offer varied answers to this question. After element 56, barium (Ba), there is an interruption between the s-block and d-block; here, one navigates to the f-block annex — also known as the lanthanoids (lanthanides) and actinoids (actinides) — under the main body. The lanthanoids and actinoids series often fall under Sc and Y, but what exact elements should belong to the same group as Sc and Y have been at the center of debate — hence the group 3 dilemma. One side of the debate puts the first two elements of the f-block, lanthanum (La) and actinium (Ac), under group 3, while the other side chooses the last two, lutetium (Lu) and lawrencium (Lr), in the block to be in group 31. The debate has slowly gained momentum over four decades. Advocates for either choice of placement have proposed various evidence to support their claim; however, the chemistry community has not yet reached an agreement to date. Though there is specific evidence that validates the placement of lanthanum and actinium, and though it is important to value chemical properties considering that is how Mendeleev first navigated periodicity, such an approach is limited in scope — leading to conflicting conclusions that are valid for both choices. Recently, more conclusive arguments that promote the Lu, Lr arrangement can resolve the exhaustive debate by applying a global perspective, emphasizing that the periodic table should be organized strictly by overall trends and principles.

Admittedly, the Sc, Y, La, Ac arrangement has a stronger historical basis; advocates for La and Ac believe that elements should be arranged based on how they react, and such belief has historical basis traced back to when the periodicity was first created. There is evidence that lanthanum does behave in more chemically similar ways to Sc and Y than lutetium does. In a study focusing on how binary compounds can inform periodic trends, Restrepo and colleagues compare over 4,700 diatomic compounds from the combination of 94 elements across the periodic table, including some lanthanides and actinides. Specifically, two elements are deemed to be chemically similar and have relational properties when they can form binary compounds with a third element. Based on the similarity landscape mapped by the study, there are more cases that La forms more similar compounds with Sc and Y than Lu does, therefore suggesting that La behaves more similar to the group 3 elements in chemical reactions and has a more rightful place in group 3. In an interview with C&EN, the study’s primary investigator Restrepo further strengthens the La, Ac argument by alluding the connection to a historical fact. He believes that behavior and reactivity should be the deciding principle in arranging the element since, historically, forerunners such as Mendeleev studied periodicity from the same approach — by researching how elements interact with each other. As stated by Restrepo, “at the core of periodic system, all you have is chemistry” when continuing building on the work of famous precedents. 

Arguments in favor of La and Ac contend to retain this principle in which specific chemical properties and similarities are prioritized. Upon close scrutinization of the elements in question for their eligibility to be put together with the d-block elements under Sc and Y, the supporters for La, Ac placement presents some reasonable explanations about the electron configurations that Lu, Lr, in comparison, would be a less favored fit. Laurence Lavelle, a faculty member of the Department of Chemistry and Biochemistry at UCLA, acutely points out that it would make no sense to keep La and Ac in the f-block regarding their valence electron configurations. For lanthanum ([Xe]5d16s2) and actinium ([Rn]6d17s2), both elements have no electrons in the f-orbital, and instead, they have one electron in the d-orbital. As a result, it would be illogical to call them f-block elements when they have no electron in that orbital at all, and Lavelle argues it can be confusing, especially for students who are learning electron configurations. 

Admittedly, this argument is immensely effective in supporting the candidacies of La and Ac, since Lu and Lr, on the other hand, have a fully filled f-orbital outside their respective noble gas cores. Nonetheless, there is equally valid proof promoting the placement of Lu and Lr on the other end of the debate. William B. Jensen’s influential 1982 article published in the Journal of Chemical Education is generally agreed upon to be the earliest in the field who addressed the group 3 debate. Contrary to the electron configuration argument proposed by Lavelle, Jensen offers convincing explanations of why La and Ac should indeed be considered f-block elements. He underlines that the d-orbital electrons of La and Ac are merely irregularities. The neighboring element, thorium (Th, [Rn]6d27s2), is of a similar case and is widely considered as an anomaly, whereas all eighteen other f-block elements conform to the idealized (n – 2)f1ns2 configuration (1982, p. 635). Consequently, the f-block should start with La and Ac as the first members and end with ytterbium (Yb) and nobelium (No), and the d-block elements in periods 6 and 7 should start with Lu and Lr. In this way, it brings an extra level of harmony that all d-block elements would all have a consistent [noble gas](n – 2)f14 core. 

As the counterevidence for Restrepo’s binary compound study, Jensen also showcases better intragroup analogies in the particular physical and chemical properties between Lu, Lr, and d-block elements. He gathered data from the works of other physicists and chemists at the time and presents the periodic trends in two tables based on various properties such as atomic radius, melting point, and electronegativity (1982, p. 635-36). Results show that Lu better conforms to the intragroup trends than La does; Lu also shares many properties — such as highest oxidation state, structure of oxide, and superconductivity — with Sc and Y, while these correlations are absent in La’s case. As a result, Jensen offers some solid validations as to why the Lu, Lr placement is an equally qualified consideration — if not better — as the La, Ac placement in the d-block.

From 2008 to 2015, multiple rounds of heated exchange between Jensen and Lavelle have drawn an increasing amount of attention to the debate. In 2015, IUPAC initiated a project to evaluate all evidence in order to eventually offer an official recommendation on which elements belong to group 3; however, five years later, the task group has not reached any conclusion. It seems that the group 3 dilemma has come to an impasse. 

However, in 2018, Eric Scerri and William Parsons propose a refreshing approach to the group 3 issue that may resolve the stalemate. This new argument emerged to conclude that solely focusing on specific chemical and physical properties of the elements can only lead to conflicting and inconclusive arguments. It should be pointed out that most arguments endeavored by both ends of the dispute to date are focused on the atomic composition, specific properties, or local trends. In this book chapter, Scerri and Parsons describe the particular chemical and physical data, which these arguments build upon, is inconclusive in nature (p. 145). For instance, both Lavelle’s and Jensen’s arguments about electron configurations are partially effective in supporting their cases and disproving their respective opponent’s, but such evidence does not help to reach an overall conclusion. Moreover, Megan Fieser, an inorganic chemistry professor at USC, stresses that the group 3 issue is complex because there are many exceptions and irregularities all over the periodic table (personal communication, April 2, 2020). This further illustrates that the scope of reasoning should not be limited locally to specific evidence since it cannot be justified globally or applied extensively for other elements or periodic trends.

Instead, Scerri and Parsons suggest a global viewpoint emphasizing overall principles and patterns based on three “modest” principal rules. They believe that the periodic table should be organized strictly by theories, overall patterns, and global characteristics that encompass all members of the periodic table rather than dwelling on specific data or electron configurations of the elements concerned. The proposal is comprised of three reasonable, fundamental requirements that can be readily accepted by most chemists (p. 145-150). First, the common 18-column format is merely a pragmatic alternative based on the 32-column long-form that presents the full periodic table; that is to say, the arrangement of elements should make sense when presented in a long-form periodic table. Second, elements should always smoothly increase in atomic number in sequence from left to right. Lastly, the arrangement should adhere to the Aufbau principle, alternatively known as the Madelung Rule, which defines the order of orbital filling. Based on these principles, Lu and Lr naturally become the more appropriate candidates for group 3 as the placement meets all three conditions with no anomalies. On the other hand, incorporating La and Ac into group 3 would result in either visually awkward splitting the d-block into two highly uneven portions meanwhile violating the Madelung Rule, or an equally undesirable disruption in the increasing order of atomic number2. Therefore, this comprehensive approach succeeds in circumventing the pitfall of never-ending clashes of evidence and providing a categorical argument in favor of the placement of Lu and Lr.

In the end, while it is important to end the debate at the moment, it is of equal importance to raise awareness of the issue. An official ruling on the group 3 debate by IUPAC will significantly benefit a large crowd from physicists and chemists to instructors and students. In the classrooms, students may find different versions of the periodic table across textbooks puzzling, and the instructors might not be able to provide a good answer. This new model of reasoning hopefully saves instructors a few awkward moments and provides them with the confidence to give a clear-cut explanation that enables students to appreciate the beauty of the periodic table. For scientists, this will mark another milestone in organizing and refining a knowledge system for the elements. In this regard, adopting the new model that emphasizes global trends and builds on general principles resonates with the notion of science as the pursuit for general, systematic truths, and hence incorporating Lu and Lr to group 3 is a justified, favored choice that would better address the dilemma. 

Select Bibliography
  1. Ball, P. (2017, April 21). The group 3 dilemma. Retrieved from https://www.chemistryworld.com/opinion/the-group-3-dilemma/3007080.article
  2. IUPAC. (2015, December 18). Project Details: The Constitution of Group 3 of the Periodic Table. Retrieved from https://iupac.org/projects/project-details/?project_nr=2015-039-2-200
  3. Jensen, W. B. (1982). The positions of lanthanum (actinium) and lutetium (lawrencium) in the periodic table, Journal of Chemical Education, 59(8), 634–636.
  4. Jensen, W. B. (2009). Misapplying the Periodic Law. Journal of Chemical Education, 86(10).
  5. Jensen, W. B. (2015). The positions of lanthanum (actinium) and lutetium (lawrencium) in the periodic table: an update. Foundations of Chemistry, 17(1), 23–31.
  6. Lavelle, L. (2008). Lanthanum (La) and Actinium (Ac) Should Remain in the d-block. Journal of Chemical Education, 85(11), 1482.
  7. Lavelle, L. (2008). Response to “The Flyleaf Periodic Table”. Journal of Chemical Education, 85(11), 1491.
  8. Lavelle, L. (2009). Response to Misapplying the Periodic Law. Journal of Chemical Education, 86(10).
  9. Lemonick, S. (2019, January 7). The periodic table is an icon. But chemists still can’t agree on how to arrange it. Chemical & Engineering News, 97(1). Retrieved from cen.acs.org/physical-chemistry/periodic-table/periodic-table-icon-chemists-still/97/i1.
  10. Restrepo, G. et al. (2012). A Network Study of Chemical Elements: From Binary Compounds to Chemical Trends. MATCH Communications in Mathamatical and in Computer Chemistry, 68, 417-442.
  11. Scerri, E. (2009). Which Elements Belong in Group 3? Journal of Chemical Education, 86(10), 1188.
  12. Scerri, E., & Parsons, W. (2018). What Elements Belong in Group 3 of the Periodic Table? In E. Scerri & G. Restrepo (Eds.), Mendeleev to Oganesson: A Multidisciplinary Perspective on the Periodic Table (pp. 140–151). New York, NY: Oxford University Press.
End Notes

1. The positions of lanthanum (La), actinium (Ac), lutetium (Lu), and lawrencium (Lr) are highlighted in the periodic table below (Retrieved from https://www.acs.org/content/acs/en/education/whatischemistry/periodictable.html). This periodic table avoids putting either La, Ac or Lu, Lr in group 3 by presenting a 15-column f-block. Many argue that although such arrangement circumvents the debate, it does not fundamentally solve the issue and can still cause confusion.

2. The figure below presents two versions of the long-form periodic table with Lu, Lr and La, Ac placement under Sc, Y. In this lower table, there are interruptions of the sequence of atomic number when navigating from Ba to Hf and Ra to Rf.

Categories
Chemistry Writings

Addressing the PFAS Action Act Debate – A Paper

Boyang (Bobby) Zhang

Should the government enact the PFAS Action Act of 2019 (H.R. 535) to address the widespread PFAS contamination?

Named “forever chemicals,” the family of per- and polyfluoroalkyl substances (PFASs) containing over 4000 chemicals has silently permeated into people’s lives since its first appearance in the 1930s. Today, their existence can be found in items ranging from grease-resistant microwave popcorn bags to firefighting foam in airports. However, along with their widespread usage, these chemicals have emerged as an urgent environmental concern over the past few years. On January 20, 2020, the controversial H.R. 535 bill, otherwise known as the PFAS Action Act of 2019, was passed by the House in hope of regulating PFASs and mitigating the public anxiety about their adverse health effects. The bill gained massive support from the public, who demands safe drinking water, and passing the bill would give the right to accessing clean water back to the people. Naturally, the urgency of the PFAS crisis — an issue that only gets worse if nothing is done — calls for immediate actions; however, the bill faces a long, rough journey ahead before being legally enacted. Many, including those in the White House, have cast doubt on the effectiveness and feasibility of the bill. In fact, the bill is criticized for failing to consider the administrative and financial consequences once it is passed as well as the reality of insufficient research and lackluster treatment solutions. Even though this issue requires an urgent response, it would be more rational to hold off legislating the bill since the negligence of the act’s real-world feasibility may result in unnecessary administrative burden and unwarranted high costs, and the bill does not fit in the current timeframe which lacks robust solutions and knowledge to tackle the PFAS problem.

To further illustrate the depth of this problem, the PFAS issue drew much public attention since the introduction of the Action Act in early 2019, which revealed an alarming urgency due to the persisting and prevailing presence of PFAS chemicals within commercial products and the environment — especially groundwater. Without strong measures taken, these chemicals would continue to accumulate in the wild. The reason why it is difficult to dispose of these chemicals is due to their molecular structure: the molecules are resistant to degradation due to the strong C–F bond interactions on the fluorocarbon chain while the polar functional group at the end makes the molecules miscible in water (514) (i). While this issue is global, what makes it most alarming for Americans is the fact that the levels of PFAS chemicals are alarmingly high in North America. In a review published on Environmental Science & Technology as early as 2006, Houde showcases that the presence of PFASs can be found in biota and living organisms around the world; however, he points out that a higher level of PFOS, a commonly studied substance in the PFAS family, in blood is found in North Americans than other continents, with the highest value of 73 ng/L found in Kentucky. More recently, major chemical manufacturers have replaced common long-chain PFASs, such as PFOS, with short-chain alternatives due to new regulations and due to the assumptions that the short-chain counterparts would lead to less bioaccumulation. Notwithstanding, in an article studying the environmental impact of short-chain PFASs, Stephan Brendel et al. strongly contend that an “effective regulation is urgently needed,” and even the presumably safer short-chains PFASs “are not of minor concern.” The study displays how the high mobility of short-chain PFASs allows them to distribute quickly into water and soil, and that they are as resistant as the long-chain counterparts to decomposition, suggesting they are, in fact, no safer than traditional PFASs. Due to the current rising usage of short-chain PFAS and its superior permeability in the environment, these warning signs further intensifies the pressing need for a set of regulations that will contain the spreading of PFAS and reduce the PFAS existence in the environment.

In addition to the adverse impacts PFAS have on the environment, the pressing health concern of PFAS is also calling for the government to take action. Several compounds in the PFAS family are confirmed to have accumulative negative effects on human health by leading to many diseases including cancer, and the PFASs have been found excessively in drinking water across the U.S. A review of the toxicity and human exposure pathways published last year by Sunderland et al. provides a holistic picture of PFAS impact on human health. The article offers evidence that six diseases such as high cholesterol, thyroid disease, and cancer are linked to PFOA, another widely studied substance in the PFAS family. Even more worrisome, the study highlights that it is more likely for children to suffer from immune issues due to PFASs (138-40). It also underlines how the occurrence of diseases increases with higher, longer exposure to accumulating and persistent PFAS chemicals. To make matters worse, the review reports about six million Americans are supplied with water higher than the advisory level released by the EPA, and there are many other alarming cases of high PFAS level in domestic drinking water (134). A study in 2012 also addresses the looming picture of PFAS contamination, that PFOS and PFOA have a synergic effect with some common pollutants such as Hg2+, Cd2+, and 2,4-D, further stressing PFAS as a severe threat to public health (Rodea-Palomares). As a result, the PFAS crisis does not allow further “delayed action” as it “has resulted in widespread human exposures and risks and lessons should be learned from this example” (Sunderland 142), and the Action Act is urgently needed to stop PFAS from entering the market.

Undeniably, a set of regulations dedicated to the PFAS issue would be helpful to alleviate some public anxiety; nonetheless, the current PFAS Action Act idealizes the solution and faces numerous challenges before being effective. While the bill expects EPA to regulate most of the PFAS family, study their toxicity, and establish drinking water standards and guidelines of disposal (H.R.535), PFAS lacks thorough research on health effects in order to properly determine a complete set of standards, and such research is both immensely time- and cost-consuming. McCarthy et al. point out four knowledge gaps in the PFAS studies. They highlight that the major data gap is “the study of the toxicity of PFAS compounds other than PFOS and PFOA.” In accordance with McCarthy, Sunderland et al. also emphasize that there is a “paucity of toxicity data” associated with “the full-diversity of individual PFASs” in their review. There is also an emerging issue that the myriad PFAS precursors, novel derivatives, and custom molecules have made the study of PFAS family increasingly tricky, while these alternative PFASs is speculated to be as equally harmful (142). As a result, the massive PFAS family, including the newly emerging members, are poorly studied at this moment, and therefore regulations and standards can hardly be made. 

Aside from omitting the lack of understanding towards the PFASs, the bill can further worsen the situation by imposing serious financial burdens on various sectors. Currently, analytical methods for PFOA and PFOS have “relatively high cost” since they require advanced equipment and trained analysts (Pontius 15). In fact, according to the budget estimate released by the Congressional Budget Office, the cost — which includes funding for PFAS research — to comply with PFAS Action Act is projected to be 715 million dollars in 2020-2024. The report also states that there are “significant uncertainties” in estimating cost spent on PFAS toxicity studies as well as the number of substances regulated. The shortage of research compared to the vast number of chemicals and the associated indefinite high cost make the Action Act hardly feasible to take effect in the near future. Furthermore, the limitations of current treatment technologies may exacerbate the budget issue — it is unknown how much time and funding would be invested before there is a superior solution. Pontius, in his comprehensive review, discusses the best available technology (BAT) for PFOA and PFOS treatment. Conventional methods chosen by water plants such as coagulation, sedimentation, filtration, oxidation, and membrane processes are found to be ineffective for PFAS removal. While some experiments found success with anion exchange and granulated activated carbon adsorption, they have major disadvantages that prevent them from being cost-efficient and utilized in large-scale (Pontius 12-14). At this moment, the uncertain timeframe of research and the lack of an effective treatment solution create a least favored situation for the act to be rushed into place, and the constraints in technology put the government in an awkward position since the limitations make it harder to comply with the regulations.

In response to the legislation of the bill, the administration released a statement in January 2020, arguing that the bill “would set problematic and unreasonable rulemaking timelines and precedents, and impose substantial, unwarranted costs.” In this regard, the statement is not wrong considering that hurried regulations would lead to unspecified responsibilities for the water suppliers, PFAS producing industries, and government agencies largely due to the variety of sources of PFAS pollution. PFAS contamination in drinking water is often associated with the use of aqueous film-forming foams (AFFF) in airports, military sites, and fire departments (Roy 3). Humans are also exposed to PFAS from myriad sources, including consumer products, air pollution, seafood, and agriculture, directly and indirectly (Sunderland 132-35) (ii). Such a complex range of sources has made it challenging to track down parties liable and effectively enforce any regulations. 

It is indeed disheartening that the PFAS Action Act cannot be enacted right away to address the urgent PFAS problem; and, understandably, a response of some sort should still take place since there are lives at stake. However, as an attempted response, the bill is hardly feasible in the sense that it requires assets that people do not possess at the moment. More importantly, it may not stop the adverse health effects since, under the pressure of these hasty regulations, companies can still get away through various loopholes. It is undesirable to blindly take actions that only provide a false promise of protection for human health. The PFAS issue is a complex one without an instant remedy. These chemicals are so prevalent and essential to people’s lives that banning the chemicals outright is simply unrealistic. Upon carefully weighing the consequences of whether passing the bill, the future would more promising if the act is held off to allow proper research to gain fuller understandings of the chemical family and to allow more time necessary for the fruition of a response that is efficient and beneficial in the long run. 

Select Bibliography

  1. Brendel, Stephan, et al. “Short-Chain Perfluoroalkyl Acids: Environmental Concerns and a Regulatory Strategy Under REACH.” Environmental Sciences Europe, vol. 30, no. 1, 2018, pp. 1–11.
  2. Buck, Robert C, et al. “Perfluoroalkyl and Polyfluoroalkyl Substances in the Environment: Terminology, Classification, and Origins.” Integrated Environmental Assessment and Management, vol. 7, no. 4, 2011, pp. 513–41.
  3. Congressional Budget Office. “S. 1507, PFAS Release Disclosure and Protection Act of 2019.” 25 Nov. 2019, https://www.cbo.gov/publication/55893. Accessed 28 Jan 2020.
  4. Houde, M, et al. “Biological Monitoring of Polyfluoroalkyl Substances: A Review.” Environmental Science & Technology, vol. 40, no. 11, 2006, pp. 3463–73.
  5. McCarthy, Chris, et al. “Ecological Considerations of Per- and Polyfluoroalkyl Substances (PFAS).” Current Pollution Reports, vol. 3, no. 4, 2017, pp. 289–301.
  6. Organisation for Economic Co-operation and Development. “Toward a New Comprehensive Global Database of Per and Polyfluoroalkyl Substances (PFASs).” OECD Environment, Health and Safety Publications, 2018, https://www.oecd.org/officialdocuments/publicdisplaydocumentpdf/?cote=ENV-JM-MONO(2018)7&doclanguage=en. Accessed 28 Jan. 2020.
  7. Pontius, Frederick. “Regulation of Perfluorooctanoic Acid (PFOA) and Perfluorooctane Sulfonic Acid (PFOS) in Drinking Water: A Comprehensive Review.” Water, vol. 11, no. 10, 2019.
  8. Rodea-Palomares, Ismael, et al. “Toxicological Interactions of Perfluorooctane Sulfonic Acid (PFOS) and Perfluorooctanoic Acid (PFOA) with Selected Pollutants.” Journal of Hazardous Materials, vol. 201-202, Elsevier B.V, Jan. 2012, pp. 209–18.
  9. Roy, Nikki D, et al. “Regulatory Challenges Posed by Emerging Contaminants.” Water Resources Committee Newsletter, vol. 20, no. 1, 2018.
  10. Sunderland, Elsie, et al. “A Review of the Pathways of Human Exposure to Poly- and Perfluoroalkyl Substances (PFASs) and Present Understanding of Health Effects.” Journal of Exposure Science and Environmental Epidemiology, vol. 29, no. 2. 2019, pp. 131–47.
  11. United States, Congress. “Summary: H.R.535.” https://www.congress.gov/bill/116th-congress/house-bill/535. Accessed 28 Jan. 2020.
  12. United States, Executive Office of the President. “Statement of Administration of Policy.” 7 Jan. 2020, https://www.whitehouse.gov/wp-content/uploads/2020/01/SAP_HR-535.pdf. Accessed 28 Jan. 2020.

Endnotes

[i] Molecular structures of PFOA and PFOS are provided below:

Perfluorooctanoic acid (PFOA)
Perfluorooctanesulfonic acid (PFOS)

[ii] From Sunderland et al. (“A Review”), the illustration describes an overview of PFAS exposure pathways for humans.