Editing Diatomic molecule (section)

== Historical significance ==
Diatomic elements played an important role in the elucidation of the concepts of element, atom, and molecule in the 19th century, because some of the most common elements, such as hydrogen, oxygen, and nitrogen, occur as diatomic molecules. [[John Dalton]]'s original atomic hypothesis assumed that all elements were monatomic and that the atoms in compounds would normally have the simplest atomic ratios with respect to one another. For example, Dalton assumed water's formula to be HO, giving the atomic weight of oxygen as eight times that of hydrogen,<ref>{{cite book|url=https://books.google.com/books?id=Bq8aDsluyggC&q=Dalton+assumed+water%27s+formula+to+be+HO%2C+giving+the+atomic+weight+of+oxygen+as+eight+times+that+of+hydrogen&pg=PA19|title=The Development of Chemical Principles|author-link1=Cooper H. Langford|last1=Langford|first1=Cooper Harold|last2=Beebe|first2=Ralph Alonzo|date=1995-01-01|publisher=Courier Corporation|isbn=9780486683591|language=en}}</ref> instead of the modern value of about 16. As a consequence, confusion existed regarding atomic weights and molecular formulas for about half a century.

As early as 1805, [[Joseph Louis Gay-Lussac|Gay-Lussac]] and [[Alexander von Humboldt|von Humboldt]] showed that water is formed of two volumes of hydrogen and one volume of oxygen, and by 1811 [[Amedeo Avogadro]] had arrived at the correct interpretation of water's composition, based on what is now called [[Avogadro's law]] and the assumption of diatomic elemental molecules. However, these results were mostly ignored until 1860, partly due to the belief that atoms of one element would have no [[chemical affinity]] toward atoms of the same element, and also partly due to apparent exceptions to Avogadro's law that were not explained until later in terms of dissociating molecules.

At the 1860 [[Karlsruhe Congress]] on atomic weights, [[Stanislao Cannizzaro|Cannizzaro]] resurrected Avogadro's ideas and used them to produce a consistent table of atomic weights, which mostly agree with modern values. These weights were an important prerequisite for the discovery of the [[periodic law]] by [[Dmitri Mendeleev]] and [[Lothar Meyer]].<ref>{{cite journal |author= Ihde, Aaron J. |title= The Karlsruhe Congress: A centennial retrospective |journal= Journal of Chemical Education |year= 1961 |volume= 38 |pages= 83–86 |url= http://search.jce.divched.org:8081/JCEIndex/FMPro?-db=jceindex.fp5&-lay=wwwform&combo=karlsruhe&-find=&-format=detail.html&-skip=0&-max=1&-token.2=0&-token.3=10 |access-date= 2007-08-24 |doi= 10.1021/ed038p83 |issue= 2 |bibcode= 1961JChEd..38...83I |archive-url= https://web.archive.org/web/20070928044137/http://search.jce.divched.org:8081/JCEIndex/FMPro?-db=jceindex.fp5&-lay=wwwform&combo=karlsruhe&-find=&-format=detail.html&-skip=0&-max=1&-token.2=0&-token.3=10 |archive-date= 28 September 2007 |url-status= dead }}</ref>