Döbereiner's triads
From Infogalactic: the planetary knowledge core
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In the history of the periodic table, Döbereiner's triads were an early attempt to sort the elements into some logical order by their physical properties. In 1829, the German chemist Johann Wolfgang Döbereiner noted that there were groups of three elements (hence "triads") which had similar physical properties. 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 near exactly predicted by taking the arithmetic mean of values for that property of the other two elements.
| Element 1 Atomic mass |
Element 2 Actual atomic mass Mean of 1 & 3 |
Element 3 Atomic mass |
|---|---|---|
| Lithium 6.9 |
Sodium 23.0 23.0 |
Potassium 39.1 |
| Calcium 40.1 |
Strontium 87.6 88.7 |
Barium 137.3 |
| Chlorine 35.5 |
Bromine 79.9 81.2 |
Iodine 126.9 |
| Sulfur 32.1 |
Selenium 79.0 79.9 |
Tellurium 127.6 |
| Carbon 12.0 |
Nitrogen 14.0 14.0 |
Oxygen 16.0 |
| Iron 55.8 |
Cobalt 58.9 57.3 |
Nickel 58.7 |
References
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