The Law of Triads was proposed by Johann Wolfgang Dobereiner, a German chemist, in the early 19th century.
If elements are arranged in increasing order of their atomic masses, the atomic mass of the middle elements of the triad is approximately the arithmetic mean of the other two.
If elements are arranged in increasing order of their atomic masses, the atomic mass of the middle elements of the triad is approximately the arithmetic mean of the other two.
| Triad Type | Elements in the Triad | (Li) | (Na) | (K) | Atomic Mass Calculation | Average |
|---|---|---|---|---|---|---|
| Alkali Metal Triad | Lithium (Li), Sodium (Na), Potassium (K) | 6.941 | 22.99 | 39.098 | (6.941+39.098)/2 = 23.02 | 23.02 |
| Alkaline Earth Metal Triad | Calcium (Ca), Strontium (Sr), Barium (Ba) | 40.078 | 87.62 | 137.327 | (40.078 + 137.327)/2 = 88.70 | 88.7 |
| Halogen Triad | Chlorine (Cl), Bromine (Br), Iodine (I) | 35.45 | 79.904 | 126.904 | (35.45 + 126.904)/ 2 = 81.18 | 81.18 |
All the known elements did not follow this rule. Law of triads was rejected as some triads nearly had the same atomic masses, e.g., (Fe, Co, Ni), (Ru, Rh, Pd), (Os, Ir, Pt)