Editing History of atomic theory (section)

==Bohr model==
{{Main|Bohr model}}

[[File:Bohr atom animation 2.gif|thumb|right|The [[Bohr model]] of the atom]]
Rutherford deduced the existence of the atomic nucleus through his experiments but he had nothing to say about how the electrons were arranged around it. In 1912, [[Niels Bohr]] joined Rutherford's lab and began his work on a quantum model of the atom.<ref name=PaisInwardBound/>{{rp|19}}

[[Max Planck]] in 1900 and  [[Albert Einstein]] in 1905 had postulated that light energy is emitted or absorbed in discrete amounts known as [[Quantum|quanta]] (singular, ''quantum''). This led to a series of atomic models with some quantum aspects, such as that of [[Arthur Erich Haas]] in 1910<ref name=PaisInwardBound/>{{rp|197}} and the 1912 [[John William Nicholson]]  atomic model with quantized angular momentum as ''h''/2{{pi}}.<ref>J. W. Nicholson, Month. Not. Roy. Astr. Soc. lxxii. pp. 49,130, 677, 693, 729 (1912).</ref><ref>The Atomic Theory of John William Nicholson, Russell McCormmach, Archive for History of Exact Sciences, Vol. 3, No. 2 (25.8.1966), pp. 160–184 (25 pages), Springer.</ref> The dynamical structure of these models was still classical, but in 1913, Bohr abandon the classical approach. He started his [[Bohr model]] of the atom with a quantum hypothesis: an electron could only orbit the nucleus in particular circular orbits with fixed [[angular momentum]] and energy, its distance from the nucleus (i.e., their radii) being proportional to its energy.<ref name=PaisInwardBound/>{{rp|197}}<ref name="NBohr">{{cite journal|author=Bohr, Niels|title=On the constitution of atoms and molecules|url=http://www.ffn.ub.es/luisnavarro/nuevo_maletin/Bohr_1913.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.ffn.ub.es/luisnavarro/nuevo_maletin/Bohr_1913.pdf |archive-date=2022-10-09 |url-status=live|journal=Philosophical Magazine|year=1913|volume=26|pages=476–502|doi=10.1080/14786441308634993|issue=153|bibcode=1913PMag...26..476B }}</ref> Under this model an electron could not lose energy in a continuous manner; instead, it could only make instantaneous "[[Atomic electron transition|quantum leap]]s" between the fixed [[energy level]]s.<ref name="NBohr"/> When this occurred, light was emitted or absorbed at a frequency proportional to the change in energy (hence the absorption and emission of light in discrete spectra).<ref name="NBohr"/>

In a trilogy of papers Bohr described and applied his model to derive the [[Balmer series]] of lines in the atomic spectrum of hydrogen and the related spectrum of He<sup>+</sup>.<ref name=PaisInwardBound/>{{rp|197}} He also used he model to describe the structure of the periodic table and aspects of chemical bonding. Together these results lead to Bohr's model being widely accepted by the end of 1915.<ref name=Kragh-2012>{{Cite book |last=Kragh |first=Helge |url=https://academic.oup.com/book/5807 |title=Niels Bohr and the Quantum Atom |date=2012-05-17 |publisher=Oxford University Press |isbn=978-0-19-965498-7 |language=en |doi=10.1093/acprof:oso/9780199654987.001.0001}}</ref>{{rp|91}}

Bohr's model was not perfect. It could only predict the [[spectral line]]s of hydrogen, not those of multielectron atoms.<ref>{{Cite journal |last=Kragh |first=Helge |author-link=Helge Kragh |date=1979 |title=Niels Bohr's Second Atomic Theory |url=https://www.jstor.org/stable/27757389 |journal=Historical Studies in the Physical Sciences |volume=10 |pages=123–186 |doi=10.2307/27757389 |jstor=27757389 |issn=0073-2672|url-access=subscription }}</ref> Worse still, it could not even account for all features of the hydrogen spectrum: as [[Spectrophotometry|spectrographic technology]] improved, it was discovered that applying a magnetic field [[Zeeman effect|caused spectral lines to multiply]] in a way that Bohr's model couldn't explain. In 1916, [[Arnold Sommerfeld]] added elliptical orbits to the Bohr model to explain the extra emission lines, but this made the model very difficult to use, and it still couldn't explain more complex atoms.<ref>{{cite book |last=Hentschel |first=Klaus |chapter=Zeeman Effect |date=2009 |chapter-url=https://link.springer.com/10.1007/978-3-540-70626-7_241 |title=Compendium of Quantum Physics |pages=862–864 |editor-last=Greenberger |editor-first=Daniel |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |language=en |doi=10.1007/978-3-540-70626-7_241 |isbn=978-3-540-70622-9 |access-date=2023-02-08 |editor2-last=Hentschel |editor2-first=Klaus |editor3-last=Weinert |editor3-first=Friedel}}</ref><ref>{{Cite journal |last=Eckert |first=Michael |date=April 2014 |title=How Sommerfeld extended Bohr's model of the atom (1913–1916) |url=http://link.springer.com/10.1140/epjh/e2013-40052-4 |journal=The European Physical Journal H |language=en |volume=39 |issue=2 |pages=141–156 |doi=10.1140/epjh/e2013-40052-4 |bibcode=2014EPJH...39..141E |s2cid=256006474 |issn=2102-6459|url-access=subscription }}</ref>