Dobereiner's Triads
Dobereiner Triads
The elements were classified on the basis of their properties. All the elements having similar properties were put in one group called a family. In the year 1829, a German chemist Dobereiner observed that certain elements had similar properties and that he could put them together in groups of three elements each. These groups of three elements were called triads. All the three elements of a particular triad had similar chemical properties. In fact, Dobereiner was the first scientist to show the relationship between the properties of elements and their atomic masses.
According to Dobereiner's law of triads :-
When elements are arranged in the order of increasing atomic masses, groups of three elements (known as triads), having similar chemical properties are obtained. The atomic mass of the middle element of the triad being equal to the arithmetic mean of the atomic masses of the other two elements.
Triad 1
The triad was made up of the Alkali Metal Group: The elements are Lithium, sodium, and potassium.
Triad Atomic Masses
Lithium 6.94
Sodium 22.99
Potassium 39.1
Mean of the masses of potassium and lithium corresponds to 23.02, which is almost equal to the atomic mass of sodium.
Triad 2
The triad was made up of the Alkaline Earth Metal Group: The elements are calcium, strontium, and barium.
Triad Atomic Masses
Calcium 40.1
Strontium 87.6
Barium 137.3
The mean of the masses of barium and calcium corresponds to 88.7.
Triad 3
The triad was made up of the Halogen Group: The elements are chlorine, bromine, and iodine.
Triad Atomic Masses
Chlorine 35.4
Bromine 79.9
Iodine 126.9
The mean value of the atomic masses of chlorine and iodine is 81.1.
Triad 4
The triad was made up of the sulfur, selenium, and tellurium.
Triad Atomic Masses
Sulfur 32.1
Selenium 78.9
Tellurium 127.6
Mean of the masses of the first and third elements in this triad corresponds to 79.85.
Triad 5
The triad was made up of Iron, cobalt and nickel constituted the last of Dobereiner’s triads.
Triad Atomic Masses
Iron 55.8
Cobalt 58.9
Nickel 58.7
Mean of the atomic masses of iron and nickel corresponds to 57.3.