Dobereiner’s classification of elements:
i. In 1829, Dobereiner classified existing elements in a tabular form by placing three elements having similar properties in a group called triad.
ii. In each triad, the elements were placed according to increasing order of their atomic masses.
iii. The atomic mass of the middle element in each triad was approximately the mean of the atomic masses of other two elements.
iv. Eg.
| Triad |
Elements |
Atomic mass |
| 1 |
Lithium (Li) |
6.9 |
| Sodium (Na) |
23 |
| Potassium (K) |
39 |
| 2 |
Calcium (Ca) |
40.1 |
| Strontium (Sr) |
87.6 |
| Barium (Ba) |
137.3 |
| 3 |
Chlorine (Cl) |
35.5 |
| Bromine (Br) |
79.9 |
| Iodine (I) |
126.9 |
| 4 |
Sulphur (S) |
32 |
| Selenium (Se) |
79 |
| Tellurium (Te) |
128 |
In the above table, 1st triad contains Li, Na and K.
Mean atomic mass of Li and K is
\(\frac{69+39}2\) = 22.95
which is approximately equal to the atomic mass of Na (23). Thus, atomic mass of Na is the approximately the mean of the atomic masses of Li and K.
v. Similarly, in the other triads, the atomic mass of the middle element is approximately the mean of the atomic masses of the other two elements.
