What are the Döbereiner Triads?

The Döbereiner triads Are groups of three chemical elements that share similar characteristics. They are part of the 118 chemical elements, being the diversity of reactions shown and its compounds, its most fascinating aspect.

The idea of ​​classifying the elements is to adequately treat their chemical properties without the need to develop in isolation a set of rules and theories for each of them.

Their periodic classification has given a systematic framework immensely useful for correlating them according to a few very simple and logical patterns.

The elements are systematically arranged in rows and columns with increasing atomic numbers, and space has been reserved for new discoveries.

In 1815 only about 30 elements were known. Although much information was available on these and their compounds, there was no apparent order.

Several attempts were made to find order, however, it was difficult to organize everything that was known, so that many scientists began to look for some pattern in their properties that would remedy this situation.

Discovery of the Triads of Döbereiner

The scientist Johann Wolfgang Döbereiner Made important findings on the numerical regularity among the atomic weights of the elements, was the first to note the existence of several groups of three elements, which he called triads, which showed chemical similarities.

These elements revealed an important numerical relation, since once ordered according to their equivalent weight, or atomic weight, the weight of the central element was to be the approximate average of the two elements remaining in the triad.

In 1817 Döbereiner found that if certain elements were combined with oxygen in binary compounds, a numerical relationship between the equivalent weights of these compounds could be discerned.

Thus, when calcium, strontium and barium oxides were considered, the equivalent weight of strontium oxide was approximately the average of calcium oxide and barium oxide. It is said that the three elements in question, strontium, calcium and barium form a triad.

Although Döbereiner worked with weights that had been deduced from experimental methods that were not rigorous, but still valid for the time, their values ​​approximate quite well when compared with the current values.

Döbereiner's observation at first had little impact on the chemical world, but then became very influential. He is now considered one of the pioneers of the development of the periodic system.

Twelve years later, in 1829, Döbereiner added three new triads, which are shown below:

Halogen Group

Chlorine, bromine and iodine have similar chemical properties and form a triad. These elements are not very reactive metals. If they are listed in order of increasing relative mass they are in order of decreasing reactivity. Bromine has an intermediate atomic mass between chlorine and iodine.

The atomic mass of the middle element Bromine (Br) is equal to the average of the atomic masses of Chlorine (Cl) and Iodine (I).

The average value obtained is close to the atomic mass of Bromine (Br).

Similarities in chemical properties:

1. They are all non-metals.
2. They all react with water to form acids (eg, in: HCl, HBr, HF).
3. All have a valence of one (for example, in: HCl, HBr, HF).
4. They all react with alkali metals to form neutral salts (eg, NaCl, NaBr, NaI)

Alkali Metals Group

Lithium, sodium and potassium have similar chemical properties and form a triad. These elements are soft metals and light but very reactive.

If they are listed in order of increasing relative atomic mass, they are also in order of increasing reactivity. Sodium has the atomic mass intermediate between lithium and potassium.

The atomic mass of the central element Sodium (Na) is equal to the average of the atomic mass of Lithium (Li) and Potassium (K).

Similarities in chemical properties:

1. They are all metals.
2. All react with water to form alkaline solutions and hydrogen gas.
3. All have a valence of one (eg, in: LiCl, NaCl, KCl).
4. Their carbonates are resistant to thermal decomposition.

Group of chalcogens or amphigenes

Sulfur, selenium and tellurium have similar chemical properties and form a triad. Selenium has the atomic mass intermediate between sulfur and tellurium.

The atomic mass of the middle element Selenium (Se) is equal to the average of atomic masses of Sulfur (S) and Teluro (Te).

Once again the average value obtained is close to the atomic mass of Selenium (Se).

Similarities in chemical properties:

1. Combinations with hydrogens of these elements result in toxic gases.
2. Each of these elements has 6 valence electrons.
3. The metallic qualities increase as the atomic number increases.

Döbereiner also warned that to be valid, triads must reveal chemical relationships between elements as well as numerical relationships.

On the other hand, he refused to group fluorine together with chlorine, bromine and iodine, as he might have done for chemical reasons, because he did not find a triadic relationship between the atomic weights of fluorine and that of these other halogens.

He was also reluctant to consider the occurrence of triads among dissimilar elements, such as nitrogen, carbon and oxygen, although they showed a significant triadic numerical relationship.

Dobereiner's work focused on the relationships between the elements of a triad, but gave no clue about the relationship between the triads.

Suffice it to say that Döbereiner's research established the notion of triads as a powerful concept, which several other chemists would soon take into consideration.

In fact, the Döbereiner triads represented the first step to group the elements into vertical columns within the periodic table and thus establish a system that explains the chemical properties and reveals the physical relationships of the elements.

Extension of Triads

Other chemists extended the Döbereiner triads to include more than three original elements. For example, fluorine was added to the top of the triad containing chlorine, bromine and iodine.

Other"triads"were produced, such as one containing oxygen, sulfur, selenium, and tellurium. But there was no system to correlate them as a whole.

One of the main drawbacks was that many relative atomic masses were still wrong for the time.

References

1. Clugston, M. and Flemming, R. (2000). Advanced Chemistry. New York, Oxford University Press.
2. Johann Wolfgang Döbereiner. Retrieved from: britannica.com.
3. Sauders, N. (2010). Breakthroughs in Science and Technology: Who Invented the Periodic Table?. Minnesota, Arcturus Publishing Limited.
4. Scerri, E. (2007). The Periodic Table: Its Story and Its Significance. New York, Oxford University Press.
5. Shyamal, A. (2008).Living Science Chemistry 10. New Delhi, Ratna Sagar P. Ltd.
6. What is group 16 of the periodic table? How are those elements used? Recovered from: quora.com.