What are Döbereiner triads?

The Döbereiner triads They 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 their compounds, their most fascinating aspect.

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

Their periodic classification has provided an immensely useful systematic framework 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 items were known. Although there was much information available on these and their compounds, there was no apparent order.

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

Discovery of the Döbereiner triads

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

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

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

Döbereiner's observation had little impact on the chemical world at first, but later became very influential. Today he is considered one of the pioneers in 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 highly reactive non-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 medium 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 (for example, in: HCl, HBr, HF).
3. They all have a valence of one (for example, in: HCl, HBr, HF).
4. All of them react with alkali metals to form neutral salts (e.g. NaCl, NaBr, NaI)

Alkali metal group

Lithium, sodium, and potassium have similar chemical properties and form a triad. These elements are soft and light metals 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 intermediate atomic mass 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. They all react with water to form alkaline solutions and hydrogen gas..
3. They all have a valence of one (for example, in: LiCl, NaCl, KCl).
4. Its carbonates are resistant to thermal decomposition.

Group of chalcogens or amphigens

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

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

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

 Similarities in chemical properties:

1. Hydrogen combinations of these elements result in toxic gases.
2. Each of these elements has 6 valence electrons.
3. Metallic qualities increase as atomic number increases.

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

On the other hand, he refused to group fluorine together with chlorine, bromine and iodine, as he could 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 appearance of triads between dissimilar elements, such as nitrogen, carbon and oxygen, despite the fact that they showed a significant triadic numerical relationship.

Dobereiner's work focused on the relationships between the elements of a triad, but did not give any clues 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 in vertical columns within the periodic table and in this way establish a system that explains the chemical properties and reveals the physical relationships of the elements..

Triads enlargement

Other chemists extended the Döbereiner triads to include more than the 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 that correlated 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. Recovered from: britannica.com.
3. Sauders, N. (2010). Breakthroughs in Science and Technology: Who Invented the Periodic Table ?. Minnesotta, 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.