Question

Describe Dobereiner’s Triad in detail

Answer 1

Answer:

Dobereiner’s Triad

Explanation:

1. During the 1820s Döbereiner’s experiments with the ignition of hydrogen on contact with powdered platinum led the Swedish chemist J.J. Berzelius to develop the concept of catalysis.
2. Toward the end of the decade Döbereiner found that the properties of bromine, a liquid, seem halfway between those of chlorine gas and the solid iodine.
3. He recalled a comparable graduation of properties in two other sequences, i.e. calcium, strontium, barium; and sulfur, selenium, tellurium.
4. Each of Dobereiner's triads was a group of three elements.
5. The appearance and reactions of the elements in a triad were similar to each other.
6. He showed that in each triad the mean of the lightest and heaviest atomic weights approximated the atomic weight of the middle element.
7. But he could not substantiate his hypothesis with a sufficient number of triads, and his findings were regarded in his time as merely interesting curiosities.
8. Dobereiner arranged the element in increasing order of atomic masses.
9. He found that the atomic mass of the middle element was approximately equal to the arithmetic mean (average) of the atomic masses of the other two elements of that triad when they are arranged in their increasing order of atomic mass.
10. 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.

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Answer 2

Answer:

The Dobereiner law of triads states the arithmetic mean of properties (like atomic weight, density) of the first and third element in a triad is nearly equal to the properties of the second element in the triad.

Explanation:

In the early 19th century, modern chemistry was under a premature state. Many elements were still unknown to the world. Also, the concept of the atomic number remained undiscovered.

The law of triads was an initial effort to classify elements based on their physical and chemical properties. The law of triads was proposed by Johann Dobereiner, a German chemist, in 1829.

In the early 1850s, Dobereiner made an attempt to group elements that shared similar properties. He arranged elements into four to five groups. Each group consisted of three elements resembling in properties. Since elements were classified into sets of three, we call the law as the law of triads. The first triad was recognized in 1817. It consisted of calcium, strontium, and barium. Other triads were found in the following years.

Dobereiner observed that properties of the middle element in a triad, especially the atomic weight and specific gravity (or density), was the averaged of the other two.

From the previous table, the atomic weight of the second element in a triad approximately equals the average of the atomic weight of other two. For example, the atomic weight of lithium and potassium is 6.9 and 39.1. The average of it gives 23.0, which is the atomic weight of sodium.

This calculation can also be extended to the density. Besides the physical properties, the elements in the triad also share chemical properties. In the first triads, all three elements are alkali metals and are placed in group 1 of the modern periodic table. They form hydroxides (LiOH, NaOH, KOH), chlorides (LiCl, NaCl, KCl), and hydrides (LiH, NaH, KH).

The same is true for the rest triads; every element in a triad shows similar physical and chemical properties.

Limitations:

The Dobereiner law of triads was an unsuccessful attempt in the classification of elements. It got shortly replaced by its successors with the discovery of new elements. It failed to fit several existing elements with similar properties into groups of three.

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