<Table> <Tr> <Td> </Td> <Td> This article may require cleanup to meet Wikipedia's quality standards . The specific problem is: Need to sync the masses and elements used here with those used in the actual publications not later modern refinements . Please help improve this article if you can . (December 2013) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article may require cleanup to meet Wikipedia's quality standards . The specific problem is: Need to sync the masses and elements used here with those used in the actual publications not later modern refinements . Please help improve this article if you can . (December 2013) (Learn how and when to remove this template message) </Td> </Tr> <P> In the 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 . By 1829, Dobereiner had found other groups of three elements (hence "triads") whose physical properties were similarly related . 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 . </P> <Table> Predicted vs actual atomic mass of the central atom of each triad <Tr> <Th> Triad name </Th> <Th_colspan="3"> Elements and atomic masses </Th> </Tr> <Tr> <Th> Element 1 Mass </Th> <Th> Element 2 Mean of 1 and 3 Actual mass </Th> <Th> Element 3 Mass </Th> </Tr> <Tr> <Td> Alkali - forming elements </Td> <Td> Lithium 6.94 </Td> <Td> Sodium 23.02 22.99 </Td> <Td> Potassium 39.10 </Td> </Tr> <Tr> <Td> Alkaline - earth - forming elements </Td> <Td> Calcium 40.1 </Td> <Td> Strontium 88.7 87.6 </Td> <Td> Barium 137.3 </Td> </Tr> <Tr> <Td> Salt - forming elements </Td> <Td> Chlorine 35.470 </Td> <Td> Bromine 80.470 78.383 </Td> <Td> Iodine 126.470 </Td> </Tr> <Tr> <Td> Acid - forming elements </Td> <Td> Sulfur 32.239 </Td> <Td> Selenium 80.741 79.263 </Td> <Td> Tellurium 129.243 </Td> </Tr> <Tr> <Td> - </Td> <Td> Iron 55.8 </Td> <Td> Cobalt 57.3 58.9 </Td> <Td> Nickel 58.7 </Td> </Tr> </Table>

Who organized known elements by placing them into groups of three called triads