History of the periodic table

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Dalton (1806): listing the known elements by atomic weight Dalton's symbols of the elements. 1806 Wellcome M0004592.jpg
Dalton (1806): listing the known elements by atomic weight
Mendeleev (1871): tabular ordering, showing periodic behavior Mendelejevs periodiska system 1871.png
Mendeleev (1871): tabular ordering, showing periodic behavior
2016: current form, 118 known elements Periodic table (18-col, enwiki), black and white.png
2016: current form, 118 known elements

The periodic table is an arrangement of the chemical elements, which are organized on the basis of their atomic numbers, electron configurations and recurring chemical properties. Elements are presented in order of increasing atomic number. The standard form of the table consists of a grid with rows called periods and columns called groups.

Periodic table Tabular arrangement of the chemical elements ordered by atomic number

The periodic table, also known as the periodic table of elements, is a tabular display of the chemical elements, which are arranged by atomic number, electron configuration, and recurring chemical properties. The structure of the table shows periodic trends. The seven rows of the table, called periods, generally have metals on the left and non-metals on the right. The columns, called groups, contain elements with similar chemical behaviours. Six groups have accepted names as well as assigned numbers: for example, group 17 elements are the halogens; and group 18 are the noble gases. Also displayed are four simple rectangular areas or blocks associated with the filling of different atomic orbitals.

Chemical element a species of atoms having the same number of protons in the atomic nucleus

A chemical element is a species of atom having the same number of protons in their atomic nuclei. For example, the atomic number of oxygen is 8, so the element oxygen consists of all atoms which have 8 protons.

Atomic number number of protons found in the nucleus of an atom

The atomic number or proton number of a chemical element is the number of protons found in the nucleus of every atom of that element. The atomic number uniquely identifies a chemical element. It is identical to the charge number of the nucleus. In an uncharged atom, the atomic number is also equal to the number of electrons.


The history of the periodic table reflects over two centuries of growth in the understanding of chemical properties, with major contributions made by Antoine-Laurent de Lavoisier, Johann Wolfgang Döbereiner, John Newlands, Julius Lothar Meyer, Dmitri Mendeleev, and Glenn T. Seaborg. [1] [2]

Johann Wolfgang Döbereiner German chemist

Johann Wolfgang Döbereiner was a German chemist who is best known for work that foreshadowed the periodic law for the chemical elements, and for inventing the first lighter, which was known as the Döbereiner's lamp. He became a professor of chemistry and pharmacy at the University of Jena.

Julius Lothar Meyer German physician and chemist

Julius Lothar Meyer was a German chemist. He was one of the pioneers in developing the first periodic table of chemical elements. Both Mendeleev and Meyer worked with Robert Bunsen. He never used his first given name, and was known throughout his life simply as Lothar Meyer.

Dmitri Mendeleev 19th and 20th-century Russian chemist

Dmitri Ivanovich Mendeleev was a Russian chemist and inventor. He is best remembered for formulating the Periodic Law and creating a farsighted version of the periodic table of elements. He used the Periodic Law not only to correct the hypothesized properties of some of the already discovered elements but also to predict the properties of eight elements that were yet to be discovered.

Early history

A number of physical elements (such as platinum, mercury, tin and zinc) have been known from antiquity, as they are found in their native form and are relatively simple to mine with primitive tools. [3] Around 330 BCE, the Greek philosopher Aristotle proposed that everything is made up of a mixture of one or more roots, an idea that had originally been suggested by the Sicilian philosopher Empedocles. The four roots, which were later renamed as elements by Plato, were earth , water , air and fire . Similar ideas about these four elements also existed in other ancient traditions, such as Indian philosophy.

Platinum Chemical element with atomic number 78

Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name is derived from the Spanish term platino, meaning "little silver".

Mercury (element) Chemical element with atomic number 80

Mercury is a chemical element with the symbol Hg and atomic number 80. It is commonly known as quicksilver and was formerly named hydrargyrum. A heavy, silvery d-block element, mercury is the only metallic element that is liquid at standard conditions for temperature and pressure; the only other element that is liquid under these conditions is the halogen bromine, though metals such as caesium, gallium, and rubidium melt just above room temperature.

Tin Chemical element with atomic number 50

Tin is a chemical element with the symbol Sn (from Latin: stannum) and atomic number 50. Tin is a silvery metal that characteristically has a faint yellow hue. Tin, like indium, is soft enough to be cut without much force. When a bar of tin is bent, the so-called tin cry can be heard as a result of sliding tin crystals reforming; this trait is shared by indium, cadmium, and frozen mercury. Pure tin after solidifying keeps a mirror-like appearance similar to most metals. However, in most tin alloys (such as pewter) the metal solidifies with a dull gray color. Tin is a post-transition metal in group 14 of the periodic table of elements. It is obtained chiefly from the mineral cassiterite, which contains stannic oxide, SnO2. Tin shows a chemical similarity to both of its neighbors in group 14, germanium and lead, and has two main oxidation states, +2 and the slightly more stable +4. Tin is the 49th most abundant element on Earth and has, with 10 stable isotopes, the largest number of stable isotopes in the periodic table, thanks to its magic number of protons. It has two main allotropes: at room temperature, the stable allotrope is β-tin, a silvery-white, malleable metal, but at low temperatures, it transforms into the less dense grey α-tin, which has the diamond cubic structure. Metallic tin does not easily oxidize in air.

Hennig Brand, as shown in The Alchemist Discovering Phosphorus Joseph Wright of Derby The Alchemist.jpg
Hennig Brand, as shown in The Alchemist Discovering Phosphorus

First categorizations

The history of the periodic table is also a history of the discovery of the chemical elements. The first person in history to discover a new element was Hennig Brand, a bankrupt German merchant. Brand tried to discover the Philosopher's Stone—a mythical object that was supposed to turn inexpensive base metals into gold. In 1669 (or later), his experiments with distilled human urine resulted in the production of a glowing white substance, which he called "cold fire" (kaltes Feuer). [4] He kept his discovery secret until 1680, when Robert Boyle rediscovered phosphorus and published his findings. The discovery of phosphorus helped to raise the question of what it meant for a substance to be an element.

Hennig Brand German merchant

Hennig Brand was a German merchant, pharmacist and alchemist, who lived and worked in Hamburg. He is the discoverer of the chemical element phosphorus.

Bankruptcy legal status of a person or other entity that cannot repay the debts it owes to creditors

Bankruptcy is a legal process through which people or other entities who cannot repay debts to creditors may seek relief from some or all of their debts. In most jurisdictions, bankruptcy is imposed by a court order, often initiated by the debtor.

Germans are a Germanic ethnic group native to Central Europe, who share a common German ancestry, culture and history. German is the shared mother tongue of a substantial majority of ethnic Germans.

In 1661, Boyle defined an element as "those primitive and simple Bodies of which the mixt ones are said to be composed, and into which they are ultimately resolved." [5]

Antoine Laurent de Lavoisier Antoine-Laurent Lavoisier (by Louis Jean Desire Delaistre)RENEW.jpg
Antoine Laurent de Lavoisier

Lavoisier's Traité Élémentaire de Chimie (Elementary Treatise of Chemistry), which was written in 1789 and first translated into English by the writer Robert Kerr, is considered to be the first modern textbook about chemistry. Lavoisier defined an element as a substance that cannot be broken down into a simpler substance by a chemical reaction. [6] This simple definition served for a century and lasted until the discovery of subatomic particles. Lavoisier's book contained a list of "simple substances" that Lavoisier believed could not be broken down further, which included oxygen, nitrogen, hydrogen, phosphorus, mercury, zinc and sulfur, which formed the basis for the modern list of elements. Lavoisier's list also included 'light' and 'caloric', which at the time were believed to be material substances. He classified these substances into metals and non metals. While many leading chemists refused to believe Lavoisier's new revelations, the Elementary Treatise was written well enough to convince the younger generation. However, Lavoisier's descriptions of his elements lack completeness, as he only classified them as metals and non-metals.

<i>Traité Élémentaire de Chimie</i> book by Antoine Lavoisier

Traité élémentaire de chimie is a textbook written by Antoine Lavoisier published in 1789 and translated into English by Robert Kerr in 1790 under the title Elements of Chemistry in a New Systematic Order containing All the Modern Discoveries. It is considered to be the first modern chemical textbook.

Robert Kerr (writer) Scottish scientific writer and translator

Dr Robert Kerr FRSE FAS FRCSE was a Scottish surgeon, writer on scientific and other subjects and translator.

Textbook educational book

A textbook is a comprehensive compilation of content in a branch of study. Textbooks are produced to meet the needs of educators, usually at educational institutions. Schoolbooks are textbooks and other books used in schools. Today, many textbooks are published in both print format and digital formats.

In 1815, the English physician and chemist William Prout noticed that atomic weights seemed to be multiples of that of hydrogen. [7]

In 1817, Johann Wolfgang Döbereiner, a chemist, began to formulate one of the earliest attempts to classify the elements. [8] 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 triads . [9]

Definition of Triad law:-"Chemically analogous elements arranged in increasing order of their atomic weights formed well marked groups of three called Triads in which the atomic weight of the middle element was found to be generally the arithmetic mean of the atomic weight of the other two elements in the triad.

  1. chlorine, bromine, and iodine
  2. calcium, strontium, and barium
  3. sulfur, selenium, and tellurium
  4. lithium, sodium, and potassium

In 1860, a revised list of elements and atomic masses was presented at a conference in Karlsruhe. It helped spur creation of more extensive systems. The first such system emerged in two years. [10]

Comprehensive formalizations

French geologist Alexandre-Emile Béguyer de Chancourtois noticed that the elements, when ordered by their atomic weights, displayed similar properties at regular intervals. In 1862, he devised a three-dimensional chart, named the "telluric helix", after the element tellurium, which fell near the center of his diagram. [11] [12] With the elements arranged in a spiral on a cylinder by order of increasing atomic weight, de Chancourtois saw that elements with similar properties lined up vertically. The original paper from Chancourtois in Comptes Rendus de l'Académie des Sciences did not include a chart and used geological rather than chemical terms. In 1863, he extended his work by including a chart and adding ions and compounds. [13]

Newlands' law of octaves Newlands periodiska system 1866.png
Newlands' law of octaves

The next attempt was made in 1864. British chemist John Newlands presented a classification of the sixty-two known elements. Newlands noticed reoccurring trends in physical properties of the elements at reoccurring intervals of multiple of eight in mass number; [14] based on this observation, he produced a classification of these elements into eight groups. Each group displayed a similar progression; Newlands likened these progressions to the progression of notes within a musical scale. [15] [16] [17] [12] Newlands's table did not leave any gaps for possible future elements and in some cases had to put two elements into the same position within the same octave. Newlands's table was ridiculed by some of his contemporaries. The Chemical Society refused to publish his work. The president of the Society William Odling defended Society's decision by saying that such 'theoretical' topics might be controversial; [18] there was even harsher opposition from within the Society, suggesting the elements could have been just as well listed alphabetically. [10] Later that year, Odling suggested a table of his own [19] but failed to get recognition following his role in opposing Newlands's table. [18]

Lothar Meyer's periodic table, published in "Die modernen Theorien der Chemie" (1864) Periodic table Meyer 1864.png
Lothar Meyer's periodic table, published in "Die modernen Theorien der Chemie" (1864)

German chemist Lothar Meyer also noted the sequences of similar chemical and physical properties repeated at periodic intervals. According to him, if the atomic weights were plotted as ordinates and the atomic volumes as abscissas—the curve obtained a series of maximums and minimums—the most electropositive elements appearing 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, classified elements into six families by their valence—for the first time, elements had been grouped according to their valence. Works on organizing the elements by atomic weight until then had been stymied by inaccurate measurements of the atomic weights. [20] In 1868, he revised his table, but this draft table was published only after his death. In a paper dated December 1869 which appeared early in 1870, Meyer published a new periodic table of 55 elements, in which the series of periods are properly ended by an element of the alkaline earth metal group. The paper also included a line chart of relative atomic volumes, which illustrated periodic relationships of physical characteristics of the elements, and which assisted Meyer in deciding where elements should appear in his periodic table. By this time he had already seen the publication of Mendeleev's first periodic table, but his work appears to have been largely independent. [3]

Dmitri Ivanovich Mendeleev Mendeleev Photographische Gesellschaft 3.jpg
Dmitri Ivanovich Mendeleev
Zeitschrift fur Chemie (1869, pages 405-6), in which Mendeleev's periodic table is first published outside Russia. Mendeleev's periodic table (1869).svg
Zeitschrift für Chemie (1869, pages 405–6), in which Mendeleev's periodic table is first published outside Russia.
Mendeleev's 1871 periodic table. Dashes: unknown elements. Group I-VII: modern group 1-7 with transition metals added; some of these extend into a group VIII. Noble gases unknown (and unpredicted). Dmitry Mendeleyev Osnovy Khimii 1869-1871 first periodic table.jpg
Mendeleev's 1871 periodic table. Dashes: unknown elements. Group I-VII: modern group 17 with transition metals added; some of these extend into a group VIII. Noble gases unknown (and unpredicted).

Russian chemist Dmitri Mendeleev arranged the elements by atomic mass, corresponding to relative molar mass. It is sometimes said that he played "chemical solitaire" on long train journeys, using cards with various facts about the known elements. [21] Another possibility is that he was inspired in part by the periodicity of the Sanskrit alphabet, which was pointed out to him by his friend an linguist Otto von Böhtlingk. [22] Mendeleev used the trends he saw to suggest that atomic weights of some elements were incorrect and accordingly changed their placing: for instance, he figured there was no place for a trivalent uranium with the mass of 120 in his work, and he doubled both the atomic weight and valency of uranium, suggesting it was a hexavalent element with the atomic weight of 240. Mendeleev also figured some spots in his ordering had no element to match, and he left gaps on account of future discoveries of these elements, using the elements before and after those missing ones to predict their properties. In 1869, he finalized his first work and had it published. [23] [24] Mendeleev also sent it to a number of well-known chemists, including Meyer; this preceded Meyer's first comprehensive periodic table which he published a few months later, acknowledging Mendeleev's priority. [25] Mendeleev continued to improve his ordering; in 1870, it gained a tabular shape, and in 1871, it was titled "periodic table". Some changes also occurred with new revisions, with some elements changing positions.

The first of Mendeleev's predictions was confirmed in 1875, when gallium was discovered; its properties were close to Mendeleev's predictions for what he termed eka-aluminium. Two more of his predictions were confirmed within another decade. [12] Mendeleev was even able to correct some initial measurements with his predictions. [26] Later chemists used this to justify Mendeleev's table. [10]


In 1882, both Meyer and Mendeleev received the Davy Medal from the Royal Society in recognition of their work on the periodic law.

The importance of Newlands' analysis was eventually recognized by the Chemistry Society with a Gold Medal five years after they recognized Mendeleev's work. It was not until the following century, with Gilbert N. Lewis's valence bond theory (1916) and Irving Langmuir's octet theory of chemical bonding (1919), that the importance of the periodicity of eight would be accepted. [27] [28] [29]

Pre-atomic theory developments

Mendeleev's 1904 table, which is an extension of his 1871 table. It includes the noble gases in group 0, and scandium, gallium, germanium, and radium are added. It has gaps in row 0 (hypothesized elements lighter than hydrogen) and row 9 (lanthanides). Mendeleev 1904 Periodic Table.png
Mendeleev's 1904 table, which is an extension of his 1871 table. It includes the noble gases in group 0, and scandium, gallium, germanium, and radium are added. It has gaps in row 0 (hypothesized elements lighter than hydrogen) and row 9 (lanthanides).
Werner's 32-column 1905 table, including many of the known lanthanides and radioactive elements. This table left spaces for many then-unknown elements, and several elements had their positions revised following advances in atomic theory. Taula periodica de Werner (1905).gif
Werner's 32-column 1905 table, including many of the known lanthanides and radioactive elements. This table left spaces for many then-unknown elements, and several elements had their positions revised following advances in atomic theory.

English chemist Henry Cavendish, the discoverer of hydrogen in 1766, discovered that air is composed of more gases than nitrogen and oxygen. [30] He recorded these findings in 1784 and 1785; among them, he found a then-unidentified gas less reactive than nitrogen. [31] Although helium was discovered in 1868, its chemistry was not investigated at that time. In 1895, William Ramsay and Lord Rayleigh isolated argon from air and determined that it was a new element. Following this discovery, Ramsay noted that an entire group of gases, the noble gases, was missing from the periodic table. Using fractional distillation to separate air, Ramsay discovered three more noble gases in 1898: neon, krypton, and xenon. Although Mendeleev's table predicted several undiscovered elements, it did not predict the existence of noble gases. Mendeleev added them to the table as Group 0 in 1902, without disturbing the basic concept of the periodic table. [31]

In addition to the predictions of scandium, gallium, and germanium that quickly were realized, Mendeleev's 1871 table left many more spaces for undiscovered elements, though he did not provide detailed predictions of their properties. In total, he predicted eighteen elements, though only half corresponded to elements that were later discovered. [26] He observed that an entire row of his table appeared to be missing between cerium and tantalum, attributing this anomaly to the presumed nature of these elements. This arrangement was used to maintain consistency with the eight-column structure, as well as the placement of the elements from osmium to gold, which were known to be analogs of ruthenium through silver. Nevertheless, he predicted the atomic weights for some of these elements, ranging from 140 for eka-molybdenum to 175 for eka-caesium. While elements with these atomic weights were later discovered, their chemistry did not correspond to the gaps in Mendeleev's table. [32] Several heavier analogs were correctly predicted, such as tri-manganese (rhenium) and dvi-iodine (astatine), though they were later found to occupy the positions of dvi-manganese and eka-iodine respectively. This imprecise use of prefixes was caused by a break in the table's structure—the apparent "dead zone" between cerium and tantalum— itself a consequence of Mendeleev's strict adherence to the eight-column structure. [32]

By 1904, Mendeleev's table rearranged several elements, and included the noble gases along with most other newly discovered elements. It still had the dead zone, and a row zero was added above hydrogen and helium to include coronium and the ether, which were widely believed to be elements at the time. [32] Although the Michelson-Morley Experiment in 1887 cast doubt on the possibility of a luminiferous ether as a space-filling medium, physicists set constraints for its properties. [33] Mendeleev believed it to be a very light gas, with an atomic weight several orders of magnitude smaller than that of hydrogen. He also postulated that it would rarely interact with other elements, similar to the noble gases of his group zero, and instead permeate substances at a velocity of 2,250 kilometres (1,400 mi) per second. [34]

In 1905, Swiss chemist Alfred Werner resolved the dead zone of Mendeleev's table. He determined that the rare earth elements (lanthanides), 13 of which were known, lay within that gap. Although Mendeleev knew of lanthanum, cerium, and erbium, they were previously unaccounted for in the table because the number of lanthanides and their exact order were not known. This was in part a consequence of their similar chemistry and imprecise determination of their atomic masses. Combined with the lack of a known group of similar elements, this rendered the placement of the lanthanides in the periodic table difficult. [35] This discovery led to a restructuring of the table and the first appearance of the 32-column form. [32]

Atomic theory and isotopes

Photographic recording of the characteristic X-ray emission lines of elements with atomic number between 20 and 29 Moseley step ladder.jpg
Photographic recording of the characteristic X-ray emission lines of elements with atomic number between 20 and 29

Four radioactive elements were known in 1900: radium, actinium, thorium, and uranium. These radioactive elements (termed "radioelements") were accordingly placed at the bottom of the periodic table, as they were known to have greater atomic weights than stable elements, although their exact order was not known. Researchers believed there were still more radioactive elements yet to be discovered, and during the next decade, the decay chains of thorium and uranium were extensively studied. Many new radioactive substances were found, including the noble gas radon, and their chemical properties were investigated. [12] By 1912, almost 50 different radioactive substances had been found in the decay chains of thorium and uranium. American chemist Bertram Boltwood proposed several decay chains linking these radioelements between uranium and lead. These were thought at the time to be new chemical elements, substantially increasing the number of known "elements" and leading to speculations that their discoveries would undermine the concept of the periodic table. [26] For example, there was not enough room between lead and uranium to accommodate these discoveries, even assuming that some discoveries were duplicates or misidentifications. It was also believed that radioactive decay violated one of the central principles of the periodic table, namely that chemical elements could not undergo transmutations and always had unique identities. [12]

Frederick Soddy and Kazimierz Fajans found in 1913 that although these substances emitted different radiation, [36] many of these substances were identical in their chemical characteristics, so shared the same place on the periodic table. [37] [38] They became known as isotopes, from the Greek isos topos ("same place"). [12] [39] Austrian chemist Friedrich Paneth cited a difference between "real elements" (elements) and "simple substances" (isotopes), also determining that the existence of different isotopes was mostly irrelevant in determining chemical properties. [26]

Following Charles Glover Barkla's discovery of characteristic X-rays emitted from metals in 1906, English physicist Henry Moseley considered a possible correlation between x-ray emissions and physical properties of elements. Moseley, along with Charles Galton Darwin, Niels Bohr, and George de Hevesy, proposed that the atomic mass (A) or nuclear charge (Z) may be mathematically related to physical properties. [40] The significance of these atomic properties was determined in the Geiger-Marsden experiment, in which the atomic nucleus and its charge were discovered. [41]

In 1913, amateur Dutch physicist Antonius van den Broek was the first to propose that the atomic number (nuclear charge) determined the placement of elements in the periodic table. He correctly determined the atomic number of all elements up to atomic number 50 (tin), though made several errors with heavier elements. However, Broek did not have any method to experimentally verify the atomic numbers of elements; thus, they were still believed to be a consequence of atomic weight, which remained in use in ordering elements. [40]

Moseley was determined to test Broek's hypothesis. [40] After a year of investigation of the Fraunhofer lines of various elements, he found a relationship between the X-ray wavelength of an element and its atomic number. [42] With this, Moseley obtained the first accurate measurements of atomic numbers and determined an absolute sequence to the elements, allowing him to restructure the periodic table. Moseley's research immediately resolved discrepancies between atomic weight and chemical properties, where sequencing strictly by atomic weight would result in groups with inconsistent chemical peroperties. For example, his measurements of X-ray wavelengths enabled him to correctly place argon (Z = 18) before potassium (Z = 19), cobalt (Z = 27) before nickel (Z = 28), as well as tellurium (Z = 52) before iodine (Z = 53), in line with periodic trends. The determination of atomic numbers also clarified the order of chemically similar rare earth elements; it was also used to confirm that Georges Urbain's claimed discovery of a new rare earth element (celtium) was invalid, earning Moseley acclamation for this technique. [40]

Swedish physicist Karl Siegbahn continued Moseley's work for elements heavier than gold (Z = 79), and found that the heaviest known element at the time, uranium, had atomic number 92. In determining the largest identified atomic number, gaps in the atomic number sequence were conclusively determined where an atomic number had no known corresponding element; the gaps occurred at atomic numbers 43, 61, 72, 75, 85, and 87. [40]

In the 1910s and 1920s, pioneering research into quantum mechanics led to new developments in atomic theory and small changes to the periodic table. The Bohr model was developed during this time, and championed the idea of electron configurations that determine chemical properties. Bohr proposed that elements in the same group behaved similarly because they have similar electron configurations, and that noble gases had filled valence shells; [43] this forms the basis of the modern octet rule. This research then led Austrian physicist Wolfgang Pauli to investigate the length of periods in the periodic table in 1924. Mendeleev asserted that there was a fixed periodicity of eight, and expected a mathematical correlation between atomic number and chemical properties; [44] Pauli demonstrated that this was not the case. Instead, the Pauli exclusion principle was developed. This states that no electrons can coexist in the same quantum state, and showed, in conjunction with empirical observations, the existence of four quantum numbers and its consequence on the order of shell filling. [43] This determines the order in which electron shells are filled and explains periodicity of the periodic table.

British chemist Charles Bury is credited with the first use of the term transition metal in 1921 to refer to elements between the main-group elements of groups II and III. He explained the chemical properties of transition elements as a consequence of the filling of an inner subshell rather than the valence shell. This proposition, based upon the work of American chemist Gilbert N. Lewis, suggested the appearance of the d subshell in period 4 and the f subshell in period 6, lengthening the periods from 8 to 18 and then 18 to 32 elements. [45]

Looking for expansions and the end of the periodic table

As early as 1913, Bohr's research on electronic structure led physicists such as Johannes Rydberg to extrapolate the properties of undiscovered elements heavier than uranium. Many agreed that the next noble gas after radon would most likely have the atomic number 118, from which it followed that the transition series in the seventh period should resemble those in the sixth. Although it was thought that these transition series would include a series analogous to the rare earth elements, characterized by filling of the 5f shell, it was unknown where this series began. Predictions ranged from atomic number 90 (thorium) to 99, many proposing a beginning beyond the known elements at or beyond atomic number 93. The elements from actinium to uranium were instead believed to form a fourth series of transition metals because of their high oxidation states; accordingly, they were placed in groups 3 through 6. [46]

In 1940, neptunium and plutonium were the first transuranic elements to be discovered; they were placed in sequence beneath rhenium and osmium, respectively. However, preliminary investigations of their chemistry suggested a greater similarity to uranium than to lighter transition metals, challenging their placement in the periodic table. [47] During his Manhattan Project research in 1943, Glenn T. Seaborg experienced unexpected difficulties in isolating the elements americium and curium, as they were believed to be part of a fourth series of transition metals. Seaborg wondered if these elements belonged to a different series, which would explain why their chemical properties, in particular the instability of higher oxidation states, were different from predictions. [47] In 1945, against the advice of colleagues, he proposed a significant change to Mendeleev's table: the actinide series. [48]

Seaborg's actinide concept of heavy element electronic structure, predicting that the actinides form a transition series analogous to the rare earth series of lanthanide elements, is now well accepted and included in the periodic table. The actinide series is the second row of the f-block (5f series), and comprises the elements from actinium to lawrencium. In both the actinide and lanthanide series, an inner electron shell is being filled.

Seaborg's subsequent elaborations of the actinide concept theorized a series of superheavy elements in a transactinide series comprising elements from 104 to 121 and a superactinide series of elements from 122 to 153. [47] He proposed an extended periodic table with an additional period of 50 elements (thus reaching element 168); this eighth period was derived from an extrapolation of the Aufbau principle and placed elements 121 to 138 in a g-block, in which a new g subshell would be filled. [49] Seaborg's model, however, did not take into account relativistic effects resulting from high atomic number and electron orbital speed. Burkhard Fricke in 1971 [50] and Pekka Pyykkö in 2010 [51] used computer modeling to calculate the positions of elements up to Z = 172, and found that the positions of several elements were different from those predicted by Seaborg. Although models from Pyykkö, Fricke, and Nefedov et al. [52] generally place element 172 as the next noble gas, there is no clear consensus on the electron configurations of elements beyond 120 and thus their placement in an extended periodic table. It is now thought that because of relativistic effects, such an extension will feature elements that break the periodicity in known elements, thus posing another hurdle to future periodic table constructs. [51]

The discovery of tennessine in 2010 filled the last remaining gap in the seventh period. Any newly discovered elements will thus be placed in an eighth period.

Despite the completion of the seventh period, experimental chemistry of some transactinides has been shown to be inconsistent with the periodic law. In the 1990s, Ken Czerwinski at University of California, Berkeley observed similarities between rutherfordium and plutonium and dubnium and protactinium, rather than a clear continuation of periodicity in groups 4 and 5. More recent experiments on copernicium and flerovium have yielded inconsistent results, some of which suggest that these elements behave more like the noble gas radon rather than mercury and lead, their respective congeners. As such, the chemistry of many superheavy elements has yet to be well-characterized, and it remains unclear whether the periodic law can still be used to extrapolate the properties of undiscovered elements. [2] [53]

See also

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An extended periodic table theorizes about chemical elements beyond those currently known in the periodic table and proven up through oganesson, which completes the seventh period (row) in the periodic table at atomic number (Z) 118.

Mendeleevs predicted elements elements predicted to exist but not yet found on the first periodic table

Dmitri Mendeleev published a periodic table of the chemical elements in 1869 based on properties that appeared with some regularity as he laid out the elements from lightest to heaviest. When Mendeleev proposed his periodic table, he noted gaps in the table and predicted that as-then-unknown elements existed with properties appropriate to fill those gaps. He named them eka-boron, eka-aluminium and eka-silicon, with respective atomic masses of 44, 68, and 72.

A period 7 element is one of the chemical elements in the seventh row of the periodic table of the chemical elements. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that elements with similar behaviour fall into the same vertical columns. The seventh period contains 32 elements, tied for the most with period 6, beginning with francium and ending with oganesson, the heaviest element currently discovered. As a rule, period 7 elements fill their 7s shells first, then their 5f, 6d, and 7p shells, in that order; however, there are exceptions, such as uranium.

John Newlands (chemist) British chemist who did work concerning the periodicity of elements

John Alexander Reina Newlands was a British chemist who worked concerning the periodicity of elements.

Group 3 element group of chemical elements

Group 3 is a group of elements in the periodic table. This group, like other d-block groups, should contain four elements, but it is not agreed what elements belong in the group. Scandium (Sc) and yttrium (Y) are always included, but the other two spaces are usually occupied by lanthanum (La) and actinium (Ac), or by lutetium (Lu) and lawrencium (Lr); less frequently, it is considered the group should be expanded to 32 elements or contracted to contain only scandium and yttrium. When the group is understood to contain all of the lanthanides, it subsumes the rare-earth metals. Yttrium, and less frequently scandium, are sometimes also counted as rare-earth metals.

History of chemistry Wikimedia history article

The history of chemistry represents a time span from ancient history to the present. By 1000 BC, civilizations used technologies that would eventually form the basis of the various branches of chemistry. Examples include extracting metals from ores, making pottery and glazes, fermenting beer and wine, extracting chemicals from plants for medicine and perfume, rendering fat into soap, making glass, and making alloys like bronze.

William Odling, FRS was an English chemist who contributed to the development of the periodic table.

In chemistry, superheavy elements, also known as transactinide elements, are the chemical elements with atomic numbers greater than 103. The superheavy elements are located immediately beyond the actinides in the periodic table; the heaviest actinide is lawrencium.

Alternative periodic tables tabulations of chemical elements differing from the traditional layout of the periodic system

Alternative periodic tables are tabulations of chemical elements differing in their organization from the traditional depiction of the periodic system.

In nuclear chemistry, the actinide concept proposed that the actinides form a second inner transition series homologous to the lanthanides. Its origins stem from observation of lanthanide-like properties in transuranic elements in contrast to the distinct complex chemistry of previously known actinides. Glenn T. Seaborg, one of the researchers who synthesized transuranic elements, proposed the actinide concept in 1944 as an explanation for observed deviations and a hypothesis to guide future experiments. It was accepted shortly thereafter, resulting in the placement of a new actinide series comprising elements 89 (actinium) to 103 (lawrencium) below the lanthanides in Dmitri Mendeleev's periodic table of the elements.

Chemistry: A Volatile History is a 2010 BBC documentary on the history of chemistry presented by Jim Al-Khalili. It was nominated for the 2010 British Academy Television Awards in the category Specialist Factual.

Chemical elements may be named from various sources: sometimes based on the person who discovered it, or the place it was discovered. Some have Latin or Greek roots deriving from something related to the element, for example some use to which it may have been put.


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