what is the Dobereiner's triads?
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Answer:
Dobereiner's triads:-
Dobereiner’s law of triads states that, the atomic mass of the middle element of a triad is the arithmetic mean of the atomic masses of the other two elements. Example: In the triad of lithium, sodium and potassium. The atomic mass of lithium is 7 and the atomic mass of potassium is 39. The average of masses of lithium and potassium gives atomic mass of sodium 23. Drawbacks: All the known elements could not be arranged in the form of triads. This law did not hold good for elements with very low or very high atomic mass.
Example: The arithmetic mean of the atomic masses of fluorine 19 and bromine 80, which comes to 49.5, varies significantly from the atomic mass of chlorine, which is 35.5.
Answer:
Dobereiner’s triads were groups of elements with similar properties that were identified by the German chemist Johann Wolfgang Dobereiner. He observed that groups of three elements (triads) could be formed in which all the elements shared similar physical and chemical properties.
Dobereiner stated in his law of triads 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. He also suggested that this law could be extended for other quantifiable properties of elements, such as density.
The first of Dobereiner’s triads was identified in the year 1817 and was constituted by the alkaline earth metals calcium, strontium, and barium. Three more triads were identified by the year 1829. These triads are tabulated below.
Triad 1
This triad was made up of the alkali metals lithium, sodium, and potassium.
Triad Atomic Masses
Lithium 6.94
Sodium 22.99
Potassium 39.1
The arithmetic mean of the masses of potassium and lithium corresponds to 23.02, which is almost equal to the atomic mass of sodium.
Triad 2
As mentioned earlier, calcium, barium, and strontium formed another one of Dobereiner’s triads.
Triad Atomic Masses
Calcium 40.1
Strontium 87.6
Barium 137.3
The mean of the masses of barium and calcium corresponds to 88.7.
Triad 3
The halogens chlorine, bromine, and iodine constituted one of the triads.
Triad Atomic Masses
Chlorine 35.4
Bromine 79.9
Iodine 126.9
The mean value of the atomic masses of chlorine and iodine is 81.1.
Triad 4
The fourth triad was formed by the elements sulfur, selenium, and tellurium.
Triad Atomic Masses
Sulfur 32.1
Selenium 78.9
Tellurium 127.6
The arithmetic mean of the masses of the first and third elements in this triad corresponds to 79.85.
Triad 5
Iron, cobalt, and nickel constituted the last of Dobereiner’s triads.
Triad Atomic Masses
Iron 55.8
Cobalt 58.9
Nickel 58.7
However, the mean of the atomic masses of iron and nickel corresponds to 57.3.