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Dobereiner's Triads: Definition, Example, limitation

In 1817, Johann Wolfgang Dobereiner, a German chemist, tried to arrange the elements with similar properties into groups. He identified certain groups of three elements. These groups of three elements having similar properties were called Dobereiner's triads after his name. He further observed that when three elements of any particular triad were arranged in order of their increasing atomic masses, the atomic mass of the middle element was roughly the mean or average of the atomic masses of the other two elements.


In fact, Dobereiner was the first scientist who showed a relationship between the properties of the elements and their atomic masses.


Examples of Dobereiner's triads.

The idea of Dobereiner's triads would be more clear from the following examples.


1. The three elements, lithium (Li), sodium (Na) and potassium (K) having similar properties constitute a Dobereiner's triad because the atomic mass of the middle element, sodium is 23 u which is roughly the average (22.95 u) of the atomic masses of lithium and potassium.


Element

Symbol

Atomic mass

Lithium

Li

6.9u

Sodium

Na

23u

Potassium

K

39.0u

Average of atomic mass of Li and K = (6.9+39.0)/2=22.95 u

Please note that the elements, lithium, sodium and potassium are collectively called alkali metals because they all react with water to form alkalis (caustic solutions).



2. The three elements, calcium (Ca), strontium (Sr) and barium (Ba) have similar properties. These elements also form a Dobereiner's triad because the atomic mass of the middle element, strontium, is 87.6 u which is very nearly the same as the average (88.7 u) of the atomic masses of calcium barium.


Element

Symbol

Atomic mass

Calcium

Ca

40.1u

Strontium

Sr

87.6u

Barium

Ba

137.3u

Average of atomic masses of Ca Ba = (40.1+137.3)/2=88.7u

Please note that the elements, calcium, strontium and barium are collectively called alkaline earth metal because their oxides are alkaline in nature and exist in the earth.



3. The three elements, chlorine (Cl), bromine (Br) and iodine (I) have similar properties. These elements constitute another Dobereiner's triad because the atomic mass of the middle element, bromine (79.9u) is roughly the average (81.2u) of the atomic masses of chlorine and iodine.


Element

Symbol

Atomic mass

Chlorine

Cl

35.5u

Bromine

Br

79.9u

Iodine

I

126.9u

Average of the atomic masses of Cl and I=(35.5+126.9)/2=81.2u

Please note that the elements, chlorine, bromine and iodine are collectively called halogens because they react with metals to form salts (halo = salt, gene =producer).



4. The three elements, sulphur (S), selenium (Se) and tellurium (Te) have similar properties. They constitute yet another Dobereiner's triad because the atomic mass of the middle element, selenium, is 78.96 u which is roughly the average (79.83 u) of the atomic masses of sulphur and tellurium.


Element

Symbol

Atomic mass

Sulphur

S

32.06u

Selenium

Se

78.96u

Tellurium

Te

127.60u

Average of the atomic masses of S and Te=(32.06+127.60)/2=79.83u

Please note that the elements, sulphur, selenium and tellurium belong to the oxygen family because the first element of this group is oxygen.



Limitations of Dobereiner's Triads

Dobereiner could identify only four triads from the elements known at that time. In other words, all the elements known at that time could not be arranged as Dobereiner's triads. For example, the three elements, nitrogen (N), phosphorus (P) and arsenic (As) have similar properties. Therefore, they can be regarded to form a triad. However, the actual atomic mass of the middle element, phosphorus (31.0 u)



Element

Symbol

Atomic mass

Nitrogen

N

14.0u

Phosphorus

P

31.0u

Arsenic

As

74.9u

Average of the atomic masses of N and As=(14.0+74.9)/2=44.45u


is much lower than the average (44.45 u) of the atomic masses of nitrogen and arsenic. Thus, these three elements do not constitute a Dobereiner's triad in spite of their similar chemical properties.


Thus, this system of classification of elements into triads could not be applied to all the elements known at that time and hence this was rejected.