09) 0) Write the postulates of Dobereiner's classification? i
Answers
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.
Explanation:
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.
Limitations of Dobereiner’s Triads
The key shortcomings of Dobereiner’s method of classifying elements are listed below.
The identification of new elements made this model obsolete.
Newly discovered elements did not fit into the triads.
Only a total of 5 Dobereiner’s triads were identified.
Even several known elements did not fit into any of the triads.
Owing to these shortcomings, other methods of classifying elements were developed.