Editing Periodic table (section)

== History ==
{{Main|History of the periodic table}}
{{See also|Timeline of chemical element discoveries}}
=== Early history ===
In 1817, German physicist [[Johann Wolfgang Döbereiner]] began to formulate one of the earliest attempts to classify the elements.<ref>{{cite journal|last1=Wurzer|first1=Ferdinand|title=Auszug eines Briefes vom Hofrath Wurzer, Prof. der Chemie zu Marburg|journal=Annalen der Physik|date=1817|volume=56|issue=7|pages=331–334|url=https://babel.hathitrust.org/cgi/pt?id=chi.096071138;view=1up;seq=351|trans-title=Excerpt of a letter from Court Advisor Wurzer, Professor of Chemistry at Marburg|language=de|doi=10.1002/andp.18170560709|bibcode=1817AnP....56..331.|access-date=15 August 2021|archive-date=8 October 2021|archive-url=https://web.archive.org/web/20211008024621/https://babel.hathitrust.org/cgi/pt?id=chi.096071138;view=1up;seq=351|url-status=live|url-access=subscription}} Here, Döbereiner found that strontium's properties were intermediate to those of calcium and barium.</ref> In 1829, he found that he could form some of the elements into groups of three, with the members of each group having related properties. He termed these groups [[Döbereiner's triads|triads]].<ref>{{cite journal|last1=Döbereiner|first1=J. W.|title=Versuch zu einer Gruppirung der elementaren Stoffe nach ihrer Analogie|journal=Annalen der Physik und Chemie|date=1829|volume=15|issue=2|pages=301–307|url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015065410634;view=1up;seq=315|series=2nd series|trans-title=An attempt to group elementary substances according to their analogies|language=de|bibcode=1829AnP....91..301D|doi=10.1002/andp.18290910217|access-date=15 August 2021|archive-date=8 October 2021|archive-url=https://web.archive.org/web/20211008024625/https://babel.hathitrust.org/cgi/pt?id=mdp.39015065410634;view=1up;seq=315|url-status=live}} For an English translation of this article, see: [https://web.lemoyne.edu/~giunta/dobereiner.html Johann Wolfgang Döbereiner: "An Attempt to Group Elementary Substances according to Their Analogies" (Lemoyne College (Syracuse, New York, USA))] {{Webarchive|url=https://web.archive.org/web/20190309161429/https://web.lemoyne.edu/~GIUNTA/dobereiner.html |date=9 March 2019 }}</ref><ref>{{cite book |last=Horvitz |first=L.|title=Eureka!: Scientific Breakthroughs That Changed The World |year=2002 |publisher=John Wiley |location=New York|isbn=978-0-471-23341-1 |oclc=50766822 |page=43|bibcode=2001esbt.book.....H}}</ref> Chlorine, bromine, and iodine formed a triad; as did calcium, strontium, and barium; lithium, sodium, and potassium; and sulfur, selenium, and tellurium. Today, all these triads form part of modern-day groups: the halogens, alkaline earth metals, alkali metals, and chalcogens.<ref>Scerri, p. 47</ref> Various chemists continued his work and were able to identify more and more relationships between small groups of elements. However, they could not build one scheme that encompassed them all.<ref>{{cite book|last=Ball|first=P.|author-link=Philip Ball|title=The Ingredients: A Guided Tour of the Elements |location=Oxford|publisher=Oxford University Press |year=2002 |isbn=978-0-19-284100-1|page=100}}</ref>

[[File:Newlands periodiska system 1866.png|thumb|right|upright=1.5|Newlands's table of the elements in 1866.|alt=Newlands's table of the elements.]]
[[John Newlands (chemist)|John Newlands]] published a letter in the ''Chemical News'' in February 1863 on the periodicity among the chemical elements.<ref name=EB1911>{{cite EB1911 |wstitle=Newlands, John Alexander Reina |volume=19 |page=515}}</ref> In 1864 Newlands published an article in the ''Chemical News'' showing that if the elements are arranged in the order of their atomic weights, those having consecutive numbers frequently either belong to the same group or occupy similar positions in different groups, and he pointed out that each eighth element starting from a given one is in this arrangement a kind of repetition of the first, like the eighth note of an octave in music (The Law of Octaves).<ref name=EB1911/> However, Newlands's formulation only worked well for the main-group elements, and encountered serious problems with the others.<ref name=jensenlaw/>

German chemist [[Lothar Meyer]] noted the sequences of similar chemical and physical properties repeated at periodic intervals. According to him, if the atomic weights were plotted as ordinates (i.e. vertically) and the atomic volumes as abscissas (i.e. horizontally)—the curve obtained a series of maximums and minimums—the most [[electropositive]] elements would appear at the peaks of the curve in the order of their atomic weights. In 1864, a book of his was published; it contained an early version of the periodic table containing 28 elements, and classified elements into six families by their [[valence (chemistry)|valence]]—for the first time, elements had been grouped according to their valence. Works on organizing the elements by atomic weight had until then been stymied by inaccurate measurements of the atomic weights.<ref name="Meyer table">Meyer, Julius Lothar; Die modernen Theorien der Chemie (1864); [https://reader.digitale-sammlungen.de/de/fs1/object/goToPage/bsb10073411.html?pageNo=147 table on page 137] {{Webarchive|url=https://web.archive.org/web/20190102050414/https://reader.digitale-sammlungen.de/de/fs1/object/goToPage/bsb10073411.html?pageNo=147 |date=2 January 2019 }}</ref> In 1868, he revised his table, but this revision was published as a draft only after his death.<ref>Scerri, pp. 106–108</ref>

=== Mendeleev ===
{{multiple image
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 | image1 = 1869-periodic-table.jpg
 | caption1 =  [[Dmitri Mendeleev|Mendeleev's]] 1869 periodic table
 | image2 = Mendelejevs periodiska system 1871.png
 | caption2 =  [[Dmitri Mendeleev|Mendeleev's]] 1871 periodic table
}}
The definitive breakthrough came from the Russian chemist [[Dmitri Mendeleev]]. Although other chemists (including Meyer) had found some other versions of the periodic system at about the same time, Mendeleev was the most dedicated to developing and defending his system, and it was his system that most affected the scientific community.<ref>Scerri, p. 113</ref> On 17 February 1869 (1 March 1869 in the Gregorian calendar), Mendeleev began arranging the elements and comparing them by their atomic weights. He began with a few elements, and over the course of the day his system grew until it encompassed most of the known elements. After he found a consistent arrangement, his printed table appeared in May 1869 in the journal of the Russian Chemical Society.<ref name="Scerri117">Scerri, pp. 117–123</ref> When elements did not appear to fit in the system, he boldly predicted that either valencies or atomic weights had been measured incorrectly, or that there was a missing element yet to be discovered.<ref name=jensenlaw/> In 1871, Mendeleev published a long article, including an updated form of his table, that made his predictions for unknown elements explicit. Mendeleev predicted the properties of three of these unknown elements in detail: as they would be missing heavier homologues of boron, aluminium, and silicon, he named them eka-boron, eka-aluminium, and eka-silicon ("eka" being Sanskrit for "one").<ref name="Scerri117" /><ref name="mendeleev1871">{{cite journal
|last1=Mendeleev
|first1=D.
|title=The natural system of elements and its application to the indication of the properties of undiscovered elements
|journal=Journal of the Russian Chemical Society
|date=1871
|volume=3
|pages=25–56
|url=https://www.knigafund.ru/books/56718/read#page31
|access-date=23 August 2017
|language=ru
|archive-date=13 August 2017
|archive-url=https://web.archive.org/web/20170813142644/https://www.knigafund.ru/books/56718/read#page31
}}</ref>{{rp|45}}
In 1875, the French chemist [[Paul-Émile Lecoq de Boisbaudran]], working without knowledge of Mendeleev's prediction, discovered a new element in a sample of the mineral [[sphalerite]], and named it gallium. He isolated the element and began determining its properties. Mendeleev, reading de Boisbaudran's publication, sent a letter claiming that gallium was his predicted eka-aluminium. Although Lecoq de Boisbaudran was initially sceptical, and suspected that Mendeleev was trying to take credit for his discovery, he later admitted that Mendeleev was correct.<ref>Scerri, p. 149</ref> In 1879, the Swedish chemist [[Lars Fredrik Nilson]] discovered a new element, which he named scandium: it turned out to be eka-boron. Eka-silicon was found in 1886 by German chemist [[Clemens Winkler]], who named it germanium. The properties of gallium, scandium, and germanium matched what Mendeleev had predicted.<ref>Scerri, p. 151–2</ref> In 1889, Mendeleev noted at the Faraday Lecture to the Royal Institution in London that he had not expected to live long enough "to mention their discovery to the Chemical Society of Great Britain as a confirmation of the exactitude and generality of the periodic law".<ref>{{cite web|last=Rouvray|first=R.|url=https://www.newscientist.com/people/dmitri-mendeleev/|title=Dmitri Mendeleev|website=New Scientist|language=en-US|access-date=19 April 2020|archive-date=15 August 2021|archive-url=https://web.archive.org/web/20210815074119/https://www.newscientist.com/people/dmitri-mendeleev/|url-status=live}}</ref> Even the discovery of the noble gases at the close of the 19th century, which Mendeleev had not predicted, fitted neatly into his scheme as an eighth main group.<ref name="Scerri164">Scerri, pp. 164–169</ref>

Mendeleev nevertheless had some trouble fitting the known lanthanides into his scheme, as they did not exhibit the periodic change in valencies that the other elements did. After much investigation, the Czech chemist [[Bohuslav Brauner]] suggested in 1902 that the lanthanides could all be placed together in one group on the periodic table. He named this the "asteroid hypothesis" as an astronomical analogy: just as there is an [[asteroid belt]] instead of a single planet between Mars and Jupiter, so the place below yttrium was thought to be occupied by all the lanthanides instead of just one element.<ref name=Thyssen />

=== Atomic number ===
[[File:Extended periodic table van den Broek.jpg|thumb|right|upright=2|Periodic table of [[Antonius van den Broek]]]]
After the internal structure of the atom was probed, amateur Dutch physicist [[Antonius van den Broek]] proposed in 1913 that the nuclear charge determined the placement of elements in the periodic table.<ref name="moseley2010">{{cite magazine |last1=Marshall |first1=J.L. |last2=Marshall |first2=V.R. |date=2010 |title=Rediscovery of the Elements: Moseley and Atomic Numbers |pages=42–47 |magazine=The Hexagon |volume=101 |issue=3 |publisher=[[Alpha Chi Sigma]] |s2cid=94398490 |url=https://pdfs.semanticscholar.org/afe4/8822cd0871e65dc5401166e7df68dc0ecb7f.pdf |access-date=15 August 2021 |archive-date=16 July 2019 |archive-url=https://web.archive.org/web/20190716215907/https://pdfs.semanticscholar.org/afe4/8822cd0871e65dc5401166e7df68dc0ecb7f.pdf }}</ref><ref>A. van den Broek, ''[[Physikalische Zeitschrift]]'', 14, (1913), 32–41</ref> The New Zealand physicist [[Ernest Rutherford]] coined the word "atomic number" for this nuclear charge.<ref>Scerri, p. 185</ref> In van den Broek's published article he illustrated the first electronic periodic table showing the elements arranged according to the number of their electrons.<ref>A. van den Broek, Die Radioelemente, das periodische System und die Konstitution der Atom, Physik. Zeitsch., 14, 32, (1913).</ref> Rutherford confirmed in his 1914 paper that Bohr had accepted the view of van den Broek.<ref>E. Rutherford, Phil. Mag., 27, 488–499 (Mar. 1914). "This has led to an interesting suggestion by van Broek that the number of units of charge on the nucleus, and consequently the number of external electrons, may be equal to the number of the elements when arranged in order of increasing atomic weight. On this view, the nucleus charges of hydrogen, helium, and carbon are 1, 2, 6 respectively, and so on for the other elements, provided there is no gap due to a missing element. This view has been taken by Bohr in his theory of the constitution of simple atoms and molecules."</ref>

The same year, English physicist [[Henry Moseley]] using [[X-ray spectroscopy]] confirmed van den Broek's proposal experimentally. Moseley determined the value of the nuclear charge of each element from [[aluminium]] to [[gold]] and showed that Mendeleev's ordering actually places the elements in sequential order by nuclear charge.<ref>{{cite book |title=The Periodic Kingdom |author=Atkins, P. W. |author-link=P. W. Atkins |publisher=HarperCollins Publishers, Inc. |year=1995 |page=[https://archive.org/details/periodickingdomj00atki/page/87 87] |isbn=978-0-465-07265-1 |url=https://archive.org/details/periodickingdomj00atki/page/87 }}</ref> Nuclear charge is identical to [[proton]] count and determines the value of the [[atomic number]] (''Z'') of each element. Using atomic number gives a definitive, integer-based sequence for the elements. Moseley's research immediately resolved discrepancies between atomic weight and chemical properties; these were cases such as tellurium and iodine, where atomic number increases but atomic weight decreases.<ref name="moseley2010" /> Although Moseley was soon killed in World War I, the Swedish physicist [[Manne Siegbahn]] continued his work up to [[uranium]], and established that it was the element with the highest atomic number then known (92).<ref>{{cite journal |last1=Egdell |first1=Russell G. |last2=Bruton |first2=Elizabeth |date=2020 |title=Henry Moseley, X-ray spectroscopy and the periodic table |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=378 |issue=2180 |doi=10.1002/chem.202004775|pmid=32811359 |doi-access=free }}</ref> Based on Moseley and Siegbahn's research, it was also known which atomic numbers corresponded to missing elements yet to be found: 43, 61, 72, 75, 85, and 87.<ref name="moseley2010" /> (Element 75 had in fact already been found by Japanese chemist [[Masataka Ogawa]] in 1908 and named ''nipponium'', but he mistakenly assigned it as element 43 instead of 75 and so his discovery was not generally recognized until later. The contemporarily accepted discovery of element 75 came in 1925, when [[Walter Noddack]], [[Ida Tacke]], and [[Otto Berg (scientist)|Otto Berg]] independently rediscovered it and gave it its present name, [[rhenium]].)<ref name=nipponium2022>{{cite journal |last1=Hisamatsu |first1=Yoji |last2=Egashira |first2=Kazuhiro |first3=Yoshiteru |last3=Maeno |date=2022 |title=Ogawa's nipponium and its re-assignment to rhenium |journal=Foundations of Chemistry |volume=24 |issue= |pages=15–57 |doi=10.1007/s10698-021-09410-x |doi-access=free }}</ref>

The dawn of atomic physics also clarified the situation of [[isotope]]s. In the [[decay chain]]s of the primordial radioactive elements thorium and uranium, it soon became evident that there were many apparent new elements that had different atomic weights but exactly the same chemical properties. In 1913, [[Frederick Soddy]] coined the term "isotope" to describe this situation, and considered isotopes to merely be different forms of the same chemical element. This furthermore clarified discrepancies such as tellurium and iodine: tellurium's natural isotopic composition is weighted towards heavier isotopes than iodine's, but tellurium has a lower atomic number.<ref name=7elements>{{cite book |last=Scerri |first=Eric |author-link= |date=2013 |title=A Tale of Seven Elements |url= |location= |publisher=Oxford University Press |pages=47–53, 115 |isbn=978-0-19-539131-2}}</ref>

=== Electron shells ===

The Danish physicist [[Niels Bohr]] applied [[Max Planck]]'s idea of quantization to the atom. He concluded that the energy levels of electrons were quantised: only a discrete set of stable energy states were allowed. Bohr then attempted to understand periodicity through electron configurations, surmising in 1913 that the inner electrons should be responsible for the chemical properties of the element.<ref>See Bohr table from 1913 paper below.</ref><ref>Helge Kragh, Aarhus, Lars Vegard, Atomic Structure, and the Periodic System, Bull. Hist. Chem., VOLUME 37, Number 1 (2012), p.43.</ref> In 1913, he produced the first electronic periodic table based on a quantum atom.<ref name="Scerri208">Scerri, pp. 208–218</ref>

Bohr called his electron shells "rings" in 1913: atomic orbitals within shells did not exist at the time of his planetary model. Bohr explains in Part 3 of his famous 1913 paper that the maximum electrons in a shell is eight, writing, "We see, further, that a ring of {{Var|n}} electrons cannot rotate in a single ring round a nucleus of charge ne unless {{Var|n}} < 8." For smaller atoms, the electron shells would be filled as follows: "rings of electrons will only join if they contain equal numbers of electrons; and that accordingly the numbers of electrons on inner rings will only be 2, 4, 8." However, in larger atoms the innermost shell would contain eight electrons: "on the other hand, the periodic system of the elements strongly suggests that already in neon {{Var|N}} = 10 an inner ring of eight electrons will occur." His proposed electron configurations for the atoms (shown to the right) mostly do not accord with those now known.<ref>Niels Bohr, "On the Constitution of Atoms and Molecules, Part III, Systems containing several nuclei" Philosophical Magazine 26:857--875 (1913)</ref><ref>{{Cite journal|last=Kragh|first=Helge|date=1 January 1979|title=Niels Bohr's Second Atomic Theory|url=https://online.ucpress.edu/hsns/article/doi/10.2307/27757389/47571/Niels-Bohr-s-Second-Atomic-Theory|journal=Historical Studies in the Physical Sciences|language=en|volume=10|pages=123–186|doi=10.2307/27757389|jstor=27757389 |issn=0073-2672|url-access=subscription}}</ref> They were improved further after the work of [[Arnold Sommerfeld]] and [[Edmund Stoner]] discovered more quantum numbers.<ref name=7elements/>

{| class="wikitable" style="float:right; font-size:95%; margin:0.5em;"
|+ Bohr's electron configurations for light elements
|-
! Element !! Electrons per shell
|-
| 4 || 2,2
|-
| 6 || 2,4
|-
| 7 || 4,3
|-
| 8 || 4,2,2
|-
| 9 || 4,4,1
|-
| 10 || 8,2
|-
| 11 || 8,2,1
|-
| 16 || 8,4,2,2
|-
| 18 || 8,8,2
|}

The first one to systematically expand and correct the chemical potentials of Bohr's atomic theory was [[Walther Kossel]] in 1914 and in 1916. Kossel explained that in the periodic table new elements would be created as electrons were added to the outer shell. In Kossel's paper, he writes: <blockquote>This leads to the conclusion that the electrons, which are added further, should be put into concentric rings or shells, on each of which ... only a certain number of electrons—namely, eight in our case—should be arranged. As soon as one ring or shell is completed, a new one has to be started for the next element; the number of electrons, which are most easily accessible, and lie at the outermost periphery, increases again from element to element and, therefore, in the formation of each new shell the chemical periodicity is repeated.<ref>W. Kossel, "Über Molekülbildung als Folge des Atom- baues", Ann. Phys., 1916, 49, 229–362 (237).</ref><ref>Translated in Helge Kragh, Aarhus, Lars Vegard, Atomic Structure, and the Periodic System, Bull. Hist. Chem., VOLUME 37, Number 1 (2012), p.43.</ref></blockquote>

In a 1919 paper, [[Irving Langmuir]] postulated the existence of "cells" which we now call orbitals, which could each only contain eight electrons each, and these were arranged in "equidistant layers" which we now call shells. He made an exception for the first shell to only contain two electrons.<ref>{{Cite journal |last=Langmuir |first=Irving |author-link=Irving Langmuir |date=June 1919 |title=The Arrangement of Electrons in Atoms and Molecules |url=https://pubs.acs.org/doi/abs/10.1021/ja02227a002 |url-status=live |journal=[[Journal of the American Chemical Society]] |language=en |volume=41 |issue=6 |pages=868–934 |doi=10.1021/ja02227a002 |bibcode=1919JAChS..41..868L |issn=0002-7863 |archive-url=https://web.archive.org/web/20210126003324/https://zenodo.org/record/1429026 |archive-date=26 January 2021 |access-date=22 October 2021|url-access=subscription }}</ref> The chemist [[Charles Rugeley Bury]] suggested in 1921 that eight and eighteen electrons in a shell form stable configurations. Bury proposed that the electron configurations in transitional elements depended upon the valence electrons in their outer shell.<ref name="Bury">{{Cite journal |last=Bury |first=Charles R. |author-link=Charles Rugeley Bury |date=July 1921 |title=Langmuir's Theory of the Arrangement of Electrons in Atoms and Molecules |url=https://pubs.acs.org/doi/abs/10.1021/ja01440a023 |url-status=live |journal=[[Journal of the American Chemical Society]] |language=en |volume=43 |issue=7 |pages=1602–1609 |doi=10.1021/ja01440a023 |bibcode=1921JAChS..43.1602B |issn=0002-7863 |archive-url=https://web.archive.org/web/20211030145903/https://zenodo.org/record/1428812 |archive-date=30 October 2021 |access-date=22 October 2021|url-access=subscription }}</ref> He introduced the word ''transition'' to describe the elements now known as [[transition metal]]s or transition elements.<ref name="Jensen2003">{{cite journal|last=Jensen|first=William B.|year=2003|title=The Place of Zinc, Cadmium, and Mercury in the Periodic Table|url=https://www.uv.es/~borrasj/ingenieria_web/temas/tema_1/lecturas_comp/p952.pdf|journal=Journal of Chemical Education|volume=80|issue=8|pages=952–961|bibcode=2003JChEd..80..952J|doi=10.1021/ed080p952|quote=The first use of the term "transition" in its modern electronic sense appears to be due to the British chemist C. R.Bury, who first used the term in his 1921 paper on the electronic structure of atoms and the periodic table|access-date=18 September 2021|archive-date=19 April 2012|archive-url=https://web.archive.org/web/20120419082806/https://www.uv.es/~borrasj/ingenieria_web/temas/tema_1/lecturas_comp/p952.pdf|url-status=live}}</ref> Bohr's theory was vindicated by the discovery of element 72: [[Georges Urbain]] claimed to have discovered it as the [[rare earth element]] ''celtium'', but Bury and Bohr had predicted that element 72 could not be a rare earth element and had to be a homologue of [[zirconium]]. [[Dirk Coster]] and [[Georg von Hevesy]] searched for the element in zirconium ores and found element 72, which they named [[hafnium]] after Bohr's hometown of [[Copenhagen]] (''Hafnia'' in Latin).<ref name="CosterHevesy1923">{{cite journal|journal = Nature|volume = 111|page=79|date=1923|doi = 10.1038/111079a0|title = On the Missing Element of Atomic Number 72|first = D.|last = Coster|author2=Hevesy, G.|issue=2777|bibcode=1923Natur.111...79C|doi-access = free}}</ref><ref>{{cite journal|title = Hafnium|url = http://www.jce.divched.org/Journal/Issues/1982/Mar/jceSubscriber/JCE1982p0242.pdf|journal = Journal of Chemical Education|last = Fernelius|first = W. C.|date = 1982|page = 242|doi = 10.1021/ed059p242|volume = 59|issue = 3|bibcode = 1982JChEd..59..242F|access-date = 3 September 2009|archive-date = 15 March 2020|archive-url = https://web.archive.org/web/20200315031648/http://www.jce.divched.org/Journal/Issues/1982/Mar/jceSubscriber/JCE1982p0242.pdf|url-status = dead}}</ref> Urbain's celtium proved to be simply purified [[lutetium]] (element 71).<ref>{{cite journal |last1=Burdette |first1=Shawn C. |last2=Thornton |first2=Brett F. |date=2018 |title=Hafnium the lutécium I used to be |url=https://www.nature.com/articles/s41557-018-0140-6 |journal=Nature Chemistry |volume=10 |issue= 10|pages=1074 |doi=10.1038/s41557-018-0140-6 |pmid=30237529 |bibcode=2018NatCh..10.1074B |access-date=8 February 2024}}</ref> Hafnium and rhenium thus became the last stable elements to be discovered.<ref name=7elements/>

Prompted by Bohr, [[Wolfgang Pauli]] took up the problem of electron configurations in 1923. Pauli extended Bohr's scheme to use four [[quantum number]]s, and formulated his [[Pauli exclusion principle|exclusion principle]] which stated that no two electrons could have the same four quantum numbers. This explained the lengths of the periods in the periodic table (2, 8, 18, and 32), which corresponded to the number of electrons that each shell could occupy.<ref name="Scerri218">Scerri, pp. 218–23</ref> In 1925, [[Friedrich Hund]] arrived at configurations close to the modern ones.<ref>{{cite journal |last1=Jensen |first1=William B. |date=2007 |title=The Origin of the s, p, d, f Orbital Labels |url=https://www.che.uc.edu/jensen/w.%20b.%20jensen/reprints/137.%20s,%20p,%20d,%20f.pdf |journal=Journal of Chemical Education |volume=84 |issue=5 |pages=757–8 |doi=10.1021/ed084p757 |bibcode=2007JChEd..84..757J |archive-url=https://web.archive.org/web/20181123140649/https://www.che.uc.edu/jensen/w.%20b.%20jensen/reprints/137.%20s,%20p,%20d,%20f.pdf |access-date=15 August 2021|archive-date=23 November 2018 }}</ref> As a result of these advances, periodicity became based on the number of chemically active or valence electrons rather than by the valences of the elements.<ref name=jensenlaw/> The [[Aufbau principle]] that describes the electron configurations of the elements was first empirically observed by [[Erwin Madelung]] in 1926,<ref name="Goudsmit">{{cite journal |title=The Order of Electron Shells in Ionized Atoms |last1=Goudsmit |first1=S. A. |last2=Richards |first2=Paul I. |journal=[[Proceedings of the National Academy of Sciences of the United States of America|Proc. Natl. Acad. Sci.]] |pages=664–671 (with correction on p&nbsp;906) |volume=51 |issue=4 |date=1964 |url=https://www.pnas.org/content/51/4/664.full.pdf |bibcode=1964PNAS...51..664G |doi=10.1073/pnas.51.4.664 |pmid=16591167 |doi-access=free |pmc=300183 |access-date=15 August 2021 |archive-date=10 October 2017 |archive-url=https://web.archive.org/web/20171010113455/https://www.pnas.org/content/51/4/664.full.pdf |url-status=live }}</ref> though the first to publish it was [[Vladimir Karapetoff]] in 1930.<ref>{{cite journal |last1=Karapetoff |first1=Vladimir |date=1930 |title=A chart of consecutive sets of electronic orbits within atoms of chemical elements |url= |journal=Journal of the Franklin Institute |volume=210 |issue=5 |pages=609–624 |doi=10.1016/S0016-0032(30)91131-3 }}</ref><ref name=Ostro>{{cite journal |last1=Ostrovsky |first1=Valentin N. |date=2003 |title=Physical Explanation of the Periodic Table |url= |journal=Annals of the New York Academy of Sciences |volume=988 |issue=1 |pages=182–192 |doi=10.1111/j.1749-6632.2003.tb06097.x |pmid=12796101 |bibcode=2003NYASA.988..182O |s2cid=21629328 }}</ref> In 1961, [[Vsevolod Klechkovsky]] derived the first part of the Madelung rule (that orbitals fill in order of increasing ''n'' + ℓ) from the [[Thomas–Fermi model]];<ref>{{cite journal |last1=Klechkovskii |first1=V.M. |title=Justification of the Rule for Successive Filling of (n+l) Groups |journal=Journal of Experimental and Theoretical Physics |date=1962 |volume=14 |issue=2 |page=334 |url=http://jetp.ras.ru/cgi-bin/e/index?t=&au=+Klechkovskii&yf=2022&yt=2022&se=1&a=s |access-date=23 June 2022}}</ref> the complete rule was derived from a similar potential in 1971 by Yury N. Demkov and Valentin N. Ostrovsky.<ref name=DO>{{cite journal |last1=Demkov |first1=Yury N. |last2=Ostrovsky |first2=Valentin N. |date=1972 |title=n+l Filling Rule in the Periodic System and Focusing Potentials |url=http://jetp.ras.ru/cgi-bin/e/index/e/35/1/p66?a=list |journal=Journal of Experimental and Theoretical Physics |volume=35 |issue=1 |pages=66–69 |doi= |bibcode=1972JETP...35...66D |access-date=25 November 2022}}</ref>{{efn|Demkov and Ostrovsky consider the potential <math>U_{1/2}(r) =  -\frac{2v}{rR(r+R)^2}</math> where <math>R</math> and <math>v</math> are constant parameters; this approaches a [[Coulomb potential]] for small <math>r</math>. When <math>v</math> satisfies the condition <math>v=v_N=\frac{1}{4}R^2 N(N+1)</math>, where <math>N=n+l</math>, the zero-energy solutions to the [[Schrödinger equation]] for this potential can be described analytically with [[Gegenbauer polynomials]]. As <math>v</math> passes through each of these values, a manifold containing all states with that value of <math>N</math> arises at zero energy and then becomes bound, recovering the Madelung order. Perturbation-theory considerations show that states with smaller <math>n</math> have lower energy, and that the s&nbsp;orbitals (with <math>l=0</math>) have their energies approaching the next <math>n+l</math> group.<ref name=DO/><ref name=shattered/>}}

[[File:Taula periòdica de Werner (1905).gif|thumb|right|512px|Periodic table of Alfred Werner (1905), the first appearance of the long form<ref name=Thyssen/>]]
The quantum theory clarified the transition metals and lanthanides as forming their own separate groups, transitional between the main groups, although some chemists had already proposed tables showing them this way before then: the English chemist Henry Bassett did so in 1892, the Danish chemist [[Julius Thomsen]] in 1895, and the Swiss chemist [[Alfred Werner]] in 1905. Bohr used Thomsen's form in his 1922 Nobel Lecture; Werner's form is very similar to the modern 32-column form. In particular, this supplanted Brauner's asteroidal hypothesis.<ref name="Thyssen">{{cite book|last1=Thyssen|first1=P.|last2=Binnemans|first2=K.|editor1-last=Gschneidner|editor1-first= K. A. Jr.|editor2-last=Bünzli|editor2-first=J-C.G|editor3-last=Vecharsky|editor3-first=Bünzli|date=2011|chapter=Accommodation of the Rare Earths in the Periodic Table: A Historical Analysis|title=Handbook on the Physics and Chemistry of Rare Earths|publisher=Elsevier|location=Amsterdam|volume=41|pages=1–93|isbn=978-0-444-53590-0|doi=10.1016/B978-0-444-53590-0.00001-7}}</ref>

The exact position of the lanthanides, and thus the composition of [[group 3 element|group 3]], remained under dispute for decades longer because their electron configurations were initially measured incorrectly.<ref name=Jensen1982/><ref name="PTSS">Scerri, pp. 392−401</ref> On chemical grounds Bassett, Werner, and Bury grouped scandium and yttrium with lutetium rather than lanthanum (the former two left an empty space below yttrium as lutetium had not yet been discovered).<ref name=Thyssen/><ref name=Bury/> Hund assumed in 1927 that all the lanthanide atoms had configuration [Xe]4f<sup>0−14</sup>5d<sup>1</sup>6s<sup>2</sup>, on account of their prevailing trivalency. It is now known that the relationship between chemistry and electron configuration is more complicated than that.{{efn|For example, the early actinides continue to behave more like the d-block transition metals in their propensity towards high oxidation states all the way from actinium to uranium, even though it is actually only actinium and thorium that have d-block-like configurations in the gas phase; f-electrons appear already at protactinium.<ref name=johnson/> Uranium's actual configuration of [Rn]5f<sup>3</sup>6d<sup>1</sup>7s<sup>2</sup> is in fact analogous to that Hund assumed for the lanthanides, but uranium does not favour the trivalent state, preferring to be tetravalent or hexavalent.<ref name=rareearths/> On the other hand, lanthanide-like configurations for the actinides begin at plutonium, but the shift towards lanthanide-like behaviour is only clear at curium: the elements between uranium and curium form a transition from transition-metal-like behaviour to lanthanide-like behaviour.<ref name=johnson/> Thus chemical behaviour and electron configuration do not exactly match each other.<ref name=johnson/>}}<ref name=rareearths>{{cite book |last1=Jørgensen |first1=Christian Klixbüll |editor1-last=Gschneidner Jr. |editor1-first=Karl A. |editor2-last=Eyring |editor2-first=Leroy |date=1988 |title=Handbook on the Physics and Chemistry of Rare Earths |publisher=Elsevier |volume=11 |pages=197–292 |chapter=Influence of Rare Earths on Chemical Understanding and Classification |isbn=978-0-444-87080-3}}</ref> Early spectroscopic evidence seemed to confirm these configurations, and thus the periodic table was structured to have group 3 as scandium, yttrium, lanthanum, and actinium, with fourteen f-elements breaking up the d-block between lanthanum and hafnium.<ref name=Jensen1982/> But it was later discovered that this is only true for four of the fifteen lanthanides (lanthanum, cerium, gadolinium, and lutetium), and that the other lanthanide atoms do not have a d-electron. In particular, ytterbium completes the 4f shell and thus Soviet physicists Lev Landau and Evgeny Lifshitz noted in 1948 that lutetium is correctly regarded as a d-block rather than an f-block element;<ref name=Landau/> that bulk lanthanum is an f-metal was first suggested by [[Jun Kondō]] in 1963, on the grounds of its low-temperature [[superconductivity]].<ref name=Kondo>{{cite journal |last1=Kondō |first1=Jun |date=January 1963 |title=Superconductivity in Transition Metals |url= |journal=Progress of Theoretical Physics |volume=29 |issue=1 |pages=1–9 |doi=10.1143/PTP.29.1 |bibcode=1963PThPh..29....1K |doi-access=free }}</ref> This clarified the importance of looking at low-lying excited states of atoms that can play a role in chemical environments when classifying elements by block and positioning them on the table.<ref name=Hamilton/><ref name=JensenLr/><ref name=Jensen1982/> Many authors subsequently rediscovered this correction based on physical, chemical, and electronic concerns and applied it to all the relevant elements, thus making group 3 contain scandium, yttrium, lutetium, and lawrencium<ref name=Hamilton/><ref name=Fluck/><ref name=PTSS/> and having lanthanum through ytterbium and actinium through nobelium as the f-block rows:<ref name=Hamilton/><ref name=Fluck/> this corrected version achieves consistency with the Madelung rule and vindicates Bassett, Werner, and Bury's initial chemical placement.<ref name=Thyssen/>

In 1988, IUPAC released a report supporting this composition of group 3,<ref name=Fluck/> a decision that was reaffirmed in 2021.<ref name="2021IUPAC">{{cite journal |last1=Scerri |first1=Eric |date=18 January 2021 |title=Provisional Report on Discussions on Group 3 of the Periodic Table |url=https://iupac.org/wp-content/uploads/2021/04/ChemInt_Jan2021_PP.pdf |journal=Chemistry International |volume=43 |issue=1 |pages=31–34 |doi=10.1515/ci-2021-0115 |s2cid=231694898 |access-date=9 April 2021 |archive-date=13 April 2021 |archive-url=https://web.archive.org/web/20210413150110/https://iupac.org/wp-content/uploads/2021/04/ChemInt_Jan2021_PP.pdf |url-status=live }}</ref> Variation can still be found in textbooks on the composition of group 3,<ref name=2015IUPAC/> and some argumentation against this format is still published today,<ref name="Jensen-2015" /> but chemists and physicists who have considered the matter largely agree on group 3 containing scandium, yttrium, lutetium, and lawrencium and challenge the counterarguments as being inconsistent.<ref name="Jensen-2015" />

=== Synthetic elements ===
[[File:Glenn Seaborg - 1964.jpg|thumb|right|Glenn T. Seaborg]]
By 1936, the pool of missing elements from hydrogen to uranium had shrunk to four: elements 43, 61, 85, and 87 remained missing. Element 43 eventually became the first element to be synthesized artificially via nuclear reactions rather than discovered in nature. It was discovered in 1937 by Italian chemists [[Emilio Segrè]] and [[Carlo Perrier]], who named their discovery [[technetium]], after the Greek word for "artificial".<ref>Scerri, pp. 313–321</ref> Elements 61 ([[promethium]]) and 85 ([[astatine]]) were likewise produced artificially in 1945 and 1940 respectively; element 87 ([[francium]]) became the last element to be discovered in nature, by French chemist [[Marguerite Perey]] in 1939.<ref>Scerri, pp. 322–340</ref>{{efn|Technetium, promethium, astatine, neptunium, and plutonium were eventually discovered to occur in nature as well, albeit in tiny traces. See [[timeline of chemical element discoveries]].}} The elements beyond uranium were likewise discovered artificially, starting with [[Edwin McMillan]] and [[Philip Abelson]]'s 1940 discovery of [[neptunium]] (via bombardment of uranium with neutrons).<ref name="Scerri354">Scerri, p. 354–6</ref> [[Glenn T. Seaborg]] and his team at the [[Lawrence Berkeley National Laboratory]] (LBNL) continued discovering transuranium elements, starting with [[plutonium]] in 1941, and discovered that contrary to previous thinking, the elements from actinium onwards were congeners of the lanthanides rather than transition metals.<ref name=Seaborg /> Bassett (1892), Werner (1905), and the French engineer [[Charles Janet]] (1928) had previously suggested this, but their ideas did not then receive general acceptance.<ref name=Thyssen /> Seaborg thus called them the actinides.<ref name="Seaborg">{{cite web |url=https://fas.org/sgp/othergov/doe/lanl/orgs/nmt/97summer.pdf |title=Source of the Actinide Concept |last=Seaborg |first=Glenn T. |date=1997 |website=fas.org |publisher=Los Alamos National Laboratory |access-date=28 March 2021 |archive-date=15 August 2021 |archive-url=https://web.archive.org/web/20210815074120/https://fas.org/sgp/othergov/doe/lanl/orgs/nmt/97summer.pdf |url-status=live }}</ref> Elements up to 101 (named mendelevium in honour of Mendeleev) were synthesized up to 1955, either through neutron or alpha-particle irradiation, or in nuclear explosions in the cases of 99 (einsteinium) and 100 (fermium).<ref name=Scerri354/>

A significant controversy arose with elements 102 through 106 in the 1960s and 1970s, as competition arose between the LBNL team (now led by [[Albert Ghiorso]]) and a team of Soviet scientists at the [[Joint Institute for Nuclear Research]] (JINR) led by [[Georgy Flyorov]]. Each team claimed discovery, and in some cases each proposed their own name for the element, creating an [[element naming controversy]] that lasted decades. These elements were made by bombardment of actinides with light ions.<ref>Scerri, pp. 356–9</ref> IUPAC at first adopted a hands-off approach, preferring to wait and see if a consensus would be forthcoming. But as it was also the height of the [[Cold War]], it became clear that this would not happen. As such, IUPAC and the [[International Union of Pure and Applied Physics]] (IUPAP) created a [[Transfermium Working Group]] (TWG, fermium being element 100) in 1985 to set out criteria for discovery,<ref>{{cite journal |last1=Öhrström |first1=Lars |last2=Holden |first2=Norman E. |date=2016 |title=The Three-letter Element Symbols |journal=Chemistry International |volume=38 |issue=2 |pages=4–8 |doi=10.1515/ci-2016-0204 |s2cid=124737708 |doi-access=free }}</ref> which were published in 1991.<ref>{{cite journal |last1=Wapstra |first1=A. H. |date=1991 |title=Criteria that must be satisfied for the discovery of a new chemical element to be recognized |url=https://old.iupac.org/reports/1991/6306wapstra/index.html |journal=Pure and Applied Chemistry |volume=63 |issue=6 |pages=879–886 |doi=10.1351/pac199163060879 |s2cid=95737691 |access-date=18 October 2022|url-access=subscription }}</ref> After some further controversy, these elements received their final names in 1997, including seaborgium (106) in honour of Seaborg.<ref>{{cite journal | doi=10.1351/pac199769122471|title=Names and symbols of transfermium elements (IUPAC Recommendations 1997) | year=1997 | journal=Pure and Applied Chemistry | volume=69 | pages=2471–2474 | issue=12| doi-access=free }}</ref>

[[File:Yuri Oganessian.jpg|thumb|right|Yuri Oganessian]]
The TWG's criteria were used to arbitrate later element discovery claims from LBNL and JINR, as well as from research institutes in Germany ([[GSI Helmholtz Centre for Heavy Ion Research|GSI]]) and Japan ([[Riken]]).<ref>{{cite journal |last1=Hofmann |first1=Sigurd |date=2019 |title=Criteria for New Element Discovery |journal=Chemistry International |volume=41 |issue=1 |pages=10–15 |doi=10.1515/ci-2019-0103|doi-access=free }}</ref> Currently, consideration of discovery claims is performed by a [[IUPAC/IUPAP Joint Working Party]]. After priority was assigned, the elements were officially added to the periodic table, and the discoverers were invited to propose their names.<ref name="IUPAC-redbook" /> By 2016, this had occurred for all elements up to 118, therefore completing the periodic table's first seven rows.<ref name="IUPAC-redbook">{{cite web |url=https://iupac.org/what-we-do/periodic-table-of-elements/ |title=Periodic Table of Elements |author=<!--Not stated--> |date=2021 |website=iupac.org |publisher=IUPAC |access-date=3 April 2021 |archive-date=10 April 2016 |archive-url=https://web.archive.org/web/20160410043726/https://iupac.org/what-we-do/periodic-table-of-elements/ |url-status=live }}</ref><ref name="finally">{{cite journal|last=Scerri|first=E.|author-link=Eric Scerri|year=2012|journal=Chemistry International|volume=34|issue=4|url=https://www.iupac.org/publications/ci/2012/3404/ud.html|title=Mendeleev's Periodic Table Is Finally Completed and What To Do about Group 3?|url-status=live|archive-url=https://web.archive.org/web/20170705051357/https://www.iupac.org/publications/ci/2012/3404/ud.html|archive-date=5 July 2017|doi=10.1515/ci.2012.34.4.28|doi-access=free}}</ref> The discoveries of elements beyond 106 were made possible by techniques devised by [[Yuri Oganessian]] at the JINR: cold fusion (bombardment of lead and bismuth by heavy ions) made possible the 1981–2004 discoveries of elements 107 through 112 at GSI and 113 at Riken, and he led the JINR team (in collaboration with American scientists) to discover elements 114 through 118 using hot fusion (bombardment of actinides by calcium ions) in 1998–2010.<ref>Scerri, pp. 356–363</ref><ref name="Chapman">{{cite journal|last1=Chapman|first1=Kit|title=What it takes to make a new element|journal=[[Chemistry World]]|date=30 November 2016|url=https://www.chemistryworld.com/what-it-takes-to-make-a-new-element/1017677.article|publisher=[[Royal Society of Chemistry]]|access-date=22 March 2022|archive-date=28 October 2017|archive-url=https://web.archive.org/web/20171028122035/https://www.chemistryworld.com/what-it-takes-to-make-a-new-element/1017677.article|url-status=live }}</ref> The heaviest known element, oganesson (118), is named in Oganessian's honour. Element 114 is named flerovium in honour of his predecessor and mentor Flyorov.<ref name=Chapman/>

In celebration of the periodic table's 150th anniversary, the [[United Nations]] declared the year 2019 as the International Year of the Periodic Table, celebrating "one of the most significant achievements in science".<ref name=":1">{{Cite news|url=https://www.bbc.com/news/science-environment-47008289|title=150 years of the periodic table: Test your knowledge |last=Briggs|first=Helen|date=29 January 2019|access-date=8 February 2019|language=en-GB|archive-url=https://web.archive.org/web/20190209210210/https://www.bbc.com/news/science-environment-47008289|archive-date=9 February 2019|url-status=live}}</ref> The discovery criteria set down by the TWG were updated in 2020 in response to experimental and theoretical progress that had not been foreseen in 1991.<ref>{{cite journal |last1=Hofmann |first1=Sigurd |last2=Dmitriev |first2=Sergey N. |last3=Fahlander |first3=Claes |last4=Gates |first4=Jacklyn M. |last5=Roberto |first5=James B. |last6=Sakai |first6=Hideyuki |date=4 August 2020 |title=On the discovery of new elements (IUPAC/IUPAP Report) |s2cid-access=free |journal=Pure and Applied Chemistry |volume=92 |issue=9 |pages=1387–1446 |doi=10.1515/pac-2020-2926 |s2cid=225377737 |doi-access=free }}</ref> Today, the periodic table is among the most recognisable icons of chemistry.<ref name="Lemonick" /> IUPAC is involved today with many processes relating to the periodic table: the recognition and naming of new elements, recommending group numbers and collective names, and the updating of atomic weights.<ref name="IUPAC-redbook" />