View source for Döbereiner's triads

[[File:Johann Wolfgang Döbereiner.jpg|thumb|Johann Wolfgang Dobereiner, who attempted to sort the elements in an order which consisted of triads.]]
In the [[Chemistry:History of the periodic table|history of the periodic table]], '''Dobereiner's triads''' were an early attempt to sort the elements into some logical order by their physical properties. In 1817, a letter reported Johann Wolfgang Dobereiner's observations of the alkaline earths; namely, that strontium had properties that were intermediate to those of calcium and barium.<ref>{{cite journal|last1=Wurzer|first1=Ferdinand|title=Auszug eines Briefes vom Hofrath Wurzer, Prof. der Chemie zu Marburg|journal=Annalen der Physik|date=1817|volume=56|pages=331–334|url=https://babel.hathitrust.org/cgi/pt?id=chi.096071138;view=1up;seq=351|trans-title=Excerpt of a letter from Court Advisor Wurzer, Professor of Chemistry at Marburg|language=German}}  From pp. 332–333:  ''"In der Gegend von Jena (bei Dornburg) … Schwerspaths seyn möchte."'' (In the area of Jena (near Dornburg) it is known that [[Chemistry:Celestine (mineral)|celestine]] has been discovered in large quantities.  This gave Mr. Dobereiner cause to inquire rigorously into the stoichiometric value of strontium oxide by a great series of experiments.  It turned out that it [i.e., the molar weight of strontium oxide] – if that of hydrogen is expressed by 1 or that of oxygen is expressed by the number 7.5 – is equal to 50.  This number is, however, precisely the arithmetic mean of that which denotes the stoichiometric value of calcium oxide (= 27.55) and of that which denotes the stoichiometric value of barium oxide (= 72.5) ; namely (27.5 + 72.5) / 2 = 50.  For a moment, Mr. Dobereiner found himself thereby caused to doubt the independent existence of strontium; however, this withstood both his analytical and synthetic experiments.  Even more noteworthy is the circumstance that the specific weight of strontium sulfide is likewise the arithmetic mean of that of pure (water-free) calcium sulfide and that [i.e., the sulfide] of barium, namely (2.9 + 4.40) / 2 = 3.65 ; which must cause [one] to believe even more that celestine might be a mixture of equal stoichiometric amounts of anhydrite [i.e., anhydrous calcium sulfate] and barite.) </ref>  By 1829, Dobereiner had found other groups of three elements (hence "triads") whose physical properties were similarly related.<ref name=D1829>{{cite journal|last1=Dobereiner|first1=J. W.|title=Versuch zu einer Gruppirung der elementaren Stoffe nach ihrer Analogie|journal=Annalen der Physik und Chemie|date=1829|volume=15|pages=301–307|url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015065410634;view=1up;seq=315|series=2nd series|trans-title=An attempt to group elementary substances according to their analogies|language=German}}  For an English translation of this article, see:  [http://web.lemoyne.edu/~giunta/dobereiner.html Johann Wolfgang Dobereiner: "An Attempt to Group Elementary Substances according to Their Analogies" (Lemoyne College (Syracuse, New York, USA))]</ref>  He also noted that some quantifiable properties of elements (e.g. atomic weight and density) in a triad followed a trend whereby the value of the middle element in the triad would be exactly or nearly predicted by taking the arithmetic mean of values for that property of the other two elements.

{| class=wikitable
|+Predicted vs actual atomic mass of the central atom of each triad
|- valign="top"
!rowspan=2 |Triad name<ref name=D1829/>
!colspan=3 |Elements and atomic masses<ref name=D1829/><ref>{{cite web | title = Johann Wolfgang Dobereiner| publisher = | date = | url = http://chemed.chem.purdue.edu/genchem/history/dobereiner.html | accessdate = 2016-03-23 |archiveurl= https://web.archive.org/web/20160323155930/http://chemed.chem.purdue.edu/genchem/history/dobereiner.html |archivedate= 2016-03-23 }}</ref>
|- valign="top"
!style=white-space:nowrap|Element 1<br>Mass
!style=white-space:nowrap|Element 2<br>[[Arithmetic mean|Mean]] of 1 and 3<br>Actual mass
!style=white-space:nowrap|Element 3<br>Mass
|- valign="top"
| Alkali-forming elements
|align="center"|[[Chemistry:Lithium|Lithium]]<br>6.94
|align="center"|[[Chemistry:Sodium|Sodium]]<br>23.02<br>22.99
|align="center"|[[Chemistry:Potassium|Potassium]]<br>39.10
|- valign="top"
| Alkaline-earth-forming elements<br>{{verify source|type=atomic masses|date=March 2018}}
|align="center"|[[Chemistry:Calcium|Calcium]]<br>40.1
|align="center"|[[Chemistry:Strontium|Strontium]]<br>88.7<br>87.6
|align="center"|[[Chemistry:Barium|Barium]]<br>137.3
|- valign="top"
| Salt-forming elements
|align="center"|[[Chemistry:Chlorine|Chlorine]]<br>35.470
|align="center"|[[Chemistry:Bromine|Bromine]]<br>80.470<br>78.383
|align="center"|[[Chemistry:Iodine|Iodine]]<br>126.470
|- valign="top"
| Acid-forming elements
|align="center"|[[Chemistry:Sulfur|Sulfur]]<br>32.239
|align="center"|[[Chemistry:Selenium|Selenium]]<br>80.741<br>79.263
|align="center"|[[Chemistry:Tellurium|Tellurium]]<br>129.243
|- valign="top"
|align="center"|[[Chemistry:Iron|Iron]]<br>55.8
|align="center"|[[Chemistry:Cobalt|Cobalt]]<br>57.3<br>58.9
|align="center"|[[Chemistry:Nickel|Nickel]]<br>58.7
|-
|}

== References ==
{{Reflist}}
* {{cite web | title = A Historic Overview: Mendeleev and the Periodic Table | publisher = | date = | url = http://genesismission.jpl.nasa.gov/educate/scimodule/UnderElem/UnderElem_pdf/HistOverST.pdf  | accessdate = 2013-01-15}}

{{DEFAULTSORT:Dobereiner's Triads}}
[[Category:History of chemistry]]
[[Category:Classification systems]]

{{Sourceattribution|Döbereiner's triads|1}}