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 pointed out that there were triads of elements that showed similar chemical properties. He also noticed the fact that the atomic weight of the central element of the triad approximately the mean of the atomic weights of the other two members. The properties of the middle element were in between those of the end members.

Element 1 Atomic mass	Element 2 Atomic mass	Element 3 Atomic mass	Arithmetic mean
Lithium 7.0	Sodium 23.0	Potassium 39.0	23.0
Calcium 40.0	Strontium 87.5	Barium 137.0	88.2
Chlorine 35.0	Bromine 80.0	Iodine 127.0	80.1
Sulfur 33.0	Selenium 79.0	Tellurium 128.0	80.0
Carbon 12.0	Nitrogen 14.0	Oxygen 16.0	14.0
Ferrum 55.8	Cobalt 58.9	Nickel 58.6	58.9

References

• "Johann Wolfgang Dobereiner". Retrieved 2013-01-15. 
• "A Historic Overview: Mendeleev and the Periodic Table". Retrieved 2013-01-15.