Döbereiner's triads
In the history of the periodic table, Döbereiner's triads were an early attempt at organizing the chemical elements by certain physical properties. In 1829, the German chemist Johann Wolfgang Döbereiner noted that there were triads of elements that showed similar chemical properties. He also noted that other properties of the elements followed trends in which the value of the middle element of the triad was in between those of the end members. For example, the atomic weight and density of the central element of the triad was at or nearly the mean of the atomic weights of the other two members. The modern periodic table has these triads of elements in adjacent spaces (either consecutive periods (rows) in a group (column) or consecutive groups in a period).
| Element 1 Atomic mass |
Element 2 Mean of 1 & 3 Actual atomic mass |
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
- "Johann Wolfgang Dobereiner". Retrieved 2013-01-15.
- "A Historic Overview: Mendeleev and the Periodic Table". Retrieved 2013-01-15.