Explain dobereiner's triad
Dobereiner's Law of Triads with Example
There have been several attempts since the beginning of time to organize the elements according to their qualities. Many theories to categorize the elements emerged when new elements were found. Various scientists supported their categorization with a variety of methods and evidence. The modern periodic table was eventually developed as a result of continuous advancements made to these hypotheses. The 18th and 19th centuries saw the discovery of a large number of new elements, rendering obsolete the general division of elements into metals and non-metals. Several tests were carried out to find and group together elements with related properties. It is crucial to remember that the formation of the contemporary periodic table was made possible by the early ways of classifying elements, such as Dobereiner's triads and Newland's law of octaves. Dobereiner triads are any of numerous groups of three chemically identical elements in which one element has an atomic weight that is almost equal to the mean of the atomic weights of the other two elements. These triads, which include the calcium-strontium-barium, sulfur-selenium-tellurium, and chlorine-bromine-iodine, were discovered between 1817 and 1829 by the German chemist J.W. Döbereiner. The oldest atomic-weight classification of these elements was this triad.
What is Dobereiner’s Triad?
Johann Wolfgang Dobereiner, a German chemist, discovered that elements with comparable properties might be arranged in sets of three or triads. A specific triad of elements had comparable chemical and physical characteristics.
Law of the Triad
Dobereiner asserts that when elements are arranged in triads, with the first and third elements having increasing atomic masses, the arithmetic mean of those atomic masses is roughly equal to the second element's atomic mass. Additionally, he suggested that this law applies to other properties of components as well. Density was one of these qualities.
Different Triad Examples
In 1817, the first Dobereiner's triad was found. It is made up of calcium, strontium, and barium, which are alkaline earth metals. Later, three further triads were found. Let's take a closer look at these triads. Dobereiner identified five triads, which are as follows:
1. First triad: The alkali metals lithium, sodium, and potassium make up this triad.
| Element | Atomic Mass |
| Lithium | 7 |
| Sodium | 23 |
| Potassium | 39 |
For this triad, the following can be used to confirm the law of triads:
The atomic mass of sodium is equal to (Lithium Atomic Mass + Potassium Atomic Mass) / 2.
M = (7 + 39)/2
= 46/2
= 23
Hence the law of the triad is proved.
2. Second triad: The halogens: Chlorine, Bromine, and Iodine made up this triad.
| Element | Atomic Mass |
| Chlorine | 35.5 |
| Bromine | 80 |
| Iodine | 127 |
For this triad, the following can be used to confirm the law of triads:
The atomic mass of Bromine is equal to (Chlorine Atomic Mass + Iodine Atomic Mass) / 2.
M = (127 + 35.5)/2
= 162.5/2
= 81.25
Hence the law of the triad is proved.
3. Third triad: Iron, cobalt, and nickel made up this triad.
| Element | Atomic Mass |
| Iron | 55.8 |
| Cobalt | 58.9 |
| Nickel | 58.7 |
For this triad, the following can be used to confirm the law of triads:
The atomic mass of cobalt is equal to (Iron Atomic Mass + Nickel Atomic Mass) / 2.
M = (58.7 + 55.8)/2
= 114.5/2
= 57.25
Hence the law of the triad is proved.
4. Fourth triad: The sulfur, selenium, and tellurium made up this triad.
| Element | Atomic Mass |
| Sulfur | 32 |
| Selenium | 79 |
| Tellurium | 128 |
For this triad, the following can be used to confirm the law of triads:
The atomic mass of selenium is equal to (sulfur Atomic Mass + tellurium Atomic Mass) / 2.
M = (32 + 128)/2
= 160/2
= 80
Hence the law of the triad is proved.
5. Fifth triad: The alkaline earth metals calcium, strontium, and barium make up this triad.
| Element | Atomic Mass |
| Calcium | 40 |
| Strontium | 87.6 |
| Barium | 137 |
For this triad, the following can be used to confirm the law of triads:
The atomic mass of strontium is equal to (calcium Atomic Mass + barium Atomic Mass) / 2.
M = (137 + 40)/2
= 177/2
= 88.5
Hence the law of the triad is proved.
Limitations
Dobereiner's triads have several drawbacks, which are mentioned below:
- A few more components did not fit into Dobereiner's triads when they were constructed.
- It led to a disorganized collection of elements.
- Only five of Dobereiner's triads could be identified, and new elements could not be grouped into them.
- The abundance of new elements discovered in the 18th and 19th centuries made it difficult and, in some cases, impossible to classify the components in Dobereiner's triads.
Dobereiner's triads laid the groundwork for the creation of the present periodic table, even though they could not arrange all elements according to their properties.