Editing Periodic table (section)

=== 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>