Triads, Dobereiner

Of the elements of Group Y B, phosphorus, arsenic and antimony constitute a Dobereiner triad. Arsenic, antimony and bismuth do not... 

Which of the Dobereiner triads shown are still listed in the same column of the modern periodic table ... 

Interestingly, as a so-called Dobereiner triad (i.e., a triad of elements, whose chemical similarities were recognized by German chemist Johann Wolfgang Dobereiner in the nineteenth century), sulfur, selenium and tellurium played a role in the initial construction of the periodic table. 

Dobereiners triads computed using atomic numbers... 

J. P. Montgomery, Dobereiner s Triads and Atomic Numbers, Journal of Chemical Education, 8-. 162-162, 1931. 

Perhaps the earliest hints of any numerical regularity among the atomic weights of the elements was discovered as early as 1817 by Dobereiner. He was the first to notice the existence of various groups of three elements, subsequently called triads, that showed chemical similarities. In addition, such elements displayed an important numerical relationship, namely that the equivalent weight, or atomic weight, of the middle element is the approximate mean of the values of the two flanking elements in the triad. 

In 1817 Dobereiner found that if certain elements were combined with oxygen in binary compounds, a numerical relationship could be discerned among the equivalent weights of these compounds. Thus when oxides of calcium, strontium, and barium were considered, the equivalent weight of strontium oxide was approximately the mean of those of calcium oxide and barium oxide. The three elements in question, strontium, calcium, and barium were said to form a triad. 

Though Dobereiner was working with weights that had been deduced with the relatively crude experimental methods of the time, his values compare rather well with current values for the triad ... 

Dobereiner s observation had little impact on the chemical world at first but later became very influential. He is now regarded as one of the earliest pioneers of the development of the periodic system. Very little happened regarding triads... 

The mean value for this triad is reasonably close to Berzelius value for bromine of 78.383. Dobereiner also obtained a triad involving some alkali metals, sodium, lithium, and potassium, which were known to share many chemical properties ... 

Suffice it to say that Dobereiner s research established the notion of triads as a powerful concept, which several other chemists were soon to take up with much effect. Indeed, Do-bereiner s triads, which would appear on the periodic table grouped in vertical columns, represented the first step in fitting the elements into a system that would account for their chemical properties and would reveal their physical relationships. 

It appears that, in the space of a single day, February 17 th 1869, Mendeleev not only began to make horizontal comparisons but also produced the first version of a full periodic table that included most of the known elements. Moreover, Mendeleev s overall approach consists of looking at atomic weight differences in conformity with the general principle of triads even though he was not specifically identifying triads in the manner of Dobereiner. 

Certainly, some similarities in the chemical properties of distant elements were recognized early on. J. W. Dobereiner was the first to draw attention to so-called "triads" (e.g. Ca, Sr, Ba). Some elements formed strong bases, others strong acids and these seemed to have an affinity for each other. 

Although many of the properties of strontia are intermediate between those of lime and baryta, he proved that it is not a combination of the two and that it bears repeated solutions, crystallizations, and precipitations without showing the smallest disposition to a separation of principles (48). Thus it is evident that Dr. Hope foreshadowed in 1793 one of the triads which J. W. Dobereiner pointed out in 1829. 

Dobereiner noticed in 1829 that there are several triads in which the middle element, that is, the one whose atomic weight lies midway between those of the other two, has properties which likewise are a mean of those of the other elements of the triad (29, 31). 

Johann Wolfgang Dobereiner, 1780-1849, Professor of chemistry at Jena. His discovery of the triads was an important step toward the systematic classification of the chemical elements He wrote many books and papers on general and pharmaceutical chemistry, mineral waters, the manufacture of vinegar, and the use of platinum as a catalyst, The original of this portrait is in the City Museum at Jena. 

Montgomery, J. P, Dobereiner s triads and atomic numbers, J. Chem Educ,... 

As the number of elements increased, so did attempts to organize them into meaningful relationships. Johann Dobereiner (1780-1849) discovered in 1829 that certain elements had atomic masses and properties that fell approximately mid-way between the masses and properties of two other elements. Dobereiner termed a set of three elements a triad. Thus, chlorine, bromine, and iodine form a triad Dobereiner proposed several other triads (lithium-sodium-potassium, calcium-strontium-barium). Dobereiner recognized that there was some sort of relationship between elements, but many elements did not fit in any triad group, and even those triads proposed displayed numerous inconsistencies. 

Ever since Lavoisier had published his list of thirty-three elements in 1789, chemists had been seeking for ways to order and classify them. Lavoisier divided the elements into gases, non-metals, metals, and earths (which included the compounds lime and magnesia). In 1829, by which time the list had expanded somewhat, Johann Wolfgang Dobereiner in Germany noticed that many elements could be grouped into threes ( triads ) whose members... 

However, other scientists had also attempted to categorise the known elements. In 1817, Johann Dobereiner noticed that the atomic weight (now called atomic mass) of strontium fell midway between the weights of calcium and barium. These were elements which possessed similar chemical properties. They formed a triad of elements. Other triads were also discovered, composed of ... 

Dobereiner found regularities in groups of three elements, triads, that had similar chemical and physical properties. 

Dobereiner s triads found a home in Mendeleyev s periodic table, published in 1872. Mendeleyev arranged the elements according to their relative weights, beginning with the lightest element, hydrogen (H). He placed elements with similar properties together vertically so that Dobereiner s triads appeared within Mendeleyev s periodic table. About... 

The elements, calcium, strontium and barium are a triad observed by Dobereiner. Below there are some properties of this triad. 

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