Editing Periodic table (section)

=== Mendeleev ===
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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 />