3.Dobereiner's Triads:
In 1829, John Dobereiner, a German Chemist, classified elements having similar properties into groups of three. These Groups are called Dobereiner's Triads. These triads have three elements whose middle one's atomic weight is the mean of the remaining two. These can be seen in the Periodic Table as groups.
| Element | Atomic Weight | Element | Atomic Weight | Element | Atomic Weight |
|---|---|---|---|---|---|
| Lithium | 7 | Calcium | 40 | Chlorine | 35.5 |
| Sodium | 23 | Strontium | 88 | Bromine | 80 |
| Potassium | 39 | Barium | 137 | Iodine | 127 |