Editing History of atomic theory (section)

==Mendeleev's periodic table==
{{main|Periodic table}}
[[Dmitrii Mendeleev]] noticed that when he arranged the elements in a row according to their atomic weights, there was a certain periodicity to them.<ref name="Scerri">{{cite book|author-link=Eric Scerri |last=Scerri |first=Eric R. |year=2020 |title=The Periodic Table, Its Story and Its Significance |edition=2nd |publisher=Oxford University Press |location=New York |isbn=978-0-190-91436-3}}</ref>{{rp|117}} For instance, the second element, [[lithium]], had similar properties to the ninth element, [[sodium]], and the sixteenth element, [[potassium]] — a period of seven. Likewise, [[beryllium]], [[magnesium]], and [[calcium]] were similar and all were seven places apart from each other on Mendeleev's table. Using these patterns, Mendeleev predicted the existence and properties of new elements, which were later discovered in nature: [[scandium]], [[gallium]], and [[germanium]].<ref name="Scerri"/>{{rp|118}} Moreover, the [[periodic table]] could predict how many atoms of other elements that an atom could bond with — e.g., germanium and carbon are in [[Carbon group|the same group on the table]] and their atoms both combine with two oxygen atoms each (GeO<sub>2</sub> and CO<sub>2</sub>). Mendeleev found these patterns validated atomic theory because it showed that the elements could be categorized by their atomic weight. Inserting a new element into the middle of a period would break the parallel between that period and the next, and would also violate Dalton's law of multiple proportions.<ref>{{cite journal|title=A Reconstruction of Development of the Periodic Table Based on History and Philosophy of Science and Its Implications for General Chemistry Textbooks |first1=Angmary |last1=Brito |first2=María A. |last2=Rodríguez |first3=Mansoor |last3=Niaz |journal=Journal of Research in Science Teaching |year=2005 |volume=42 |number=1 |pages=84–111 |doi=10.1002/tea.20044|bibcode=2005JRScT..42...84B }}</ref>
[[File:Mendelejevs periodiska system 1871.png|center|thumb|upright=4|Mendeleev's [[periodic table]] from 1871.]]

The elements on the [[periodic table]] were originally arranged in order of increasing atomic weight. However, in a number of places chemists chose to swap the positions of certain adjacent elements so that they appeared in a group with other elements with similar properties. For instance, [[tellurium]] is placed before [[iodine]] even though tellurium is heavier (127.6 vs 126.9) so that iodine can be in the same column as the other [[halogens]]. The modern periodic table is based on [[atomic number]], which is equivalent to the nuclear charge, a change had to wait for the [[#Discovery of the nucleus|discovery of the nucleus]].<ref name="PaisInwardBound"/>{{rp|228}} 
In addition, an entire row of the table was not shown
because the [[noble gas]]es had not been discovered when Mendeleev devised his table.<ref name="PaisInwardBound"/>{{rp|222}}