Dobereiner's rules, also known as the Law of Triads, describe a pattern he observed among certain groups of three elements. The core rule is based on the atomic masses of these elements.
The Law of Triads Explained
The key idea of Dobereiner's Law of Triads is that within a group of three similar elements (a triad), the atomic mass of the middle element is approximately the average of the atomic masses of the first and third elements.
- Atomic Mass Relationship: The reference states that "the arithmetic mean of the atomic masses of the first and third element in a triad would be approximately equal to the atomic mass of the second element in that triad".
How the Law Works in Practice
Dobereiner identified several triads where this pattern holds true. Let's visualize this using a table:
| Triad | Element 1 | Atomic Mass (approx.) | Element 2 | Atomic Mass (approx.) | Element 3 | Atomic Mass (approx.) | Calculated Average (E1 & E3) |
|---|---|---|---|---|---|---|---|
| Lithium | Lithium | 7 | Sodium | 23 | Potassium | 39 | 23 |
| Calcium | Calcium | 40 | Strontium | 88 | Barium | 137 | 88.5 |
| Chlorine | Chlorine | 35.5 | Bromine | 80 | Iodine | 127 | 81.25 |
| Sulfur | Sulfur | 32 | Selenium | 79 | Tellurium | 128 | 80 |
- Calculated Average: As you can see in the table, the atomic mass of the middle element (Element 2) is close to the average of the atomic masses of the first (Element 1) and third (Element 3) elements.
Limitations of Dobereiner's Law
While significant for its time, Dobereiner's Law of Triads had limitations:
- Not Universal: It didn't apply to all known elements. Many elements did not fit into the triad model.
- Limited Groups: Only a few groups of elements were found to follow this rule.
- Inconsistencies: The averaging wasn't perfect; there were often slight discrepancies in the calculated average compared to actual atomic mass of the middle element.
Conclusion
Dobereiner's Law of Triads, although limited in its scope, was an important early attempt to categorize elements and identify patterns in their properties. It laid the groundwork for later, more comprehensive organization of the elements, such as the periodic table. The rule states that within a triad, the arithmetic mean of the atomic masses of the first and third elements is approximately equal to the atomic mass of the middle element.
