During 1817–1829, Johann Dobereiner began to associate elements with similar properties in to groups
of three or triads. This began in 1817 when he noticed that the atomic weights of strontium, calcium and barium. Sr, strontium was
found to be in midway between the weights of calcium and barium. All the three elements possessed similar chemical properties. By
1829, he had discovered a halogen triad made up of chlorine, bromine, and iodine and a alkali metal triad of lithium, sodium and
potassium. He postulated that nature contained triads of elements in which the middle element had properties that were an average
of the other two elements. Later, other scientists found other triads and recognized that elements could be grouped into set large
than three. The poor accuracy of measurements such as that of atomic weights hindered grouping more elements and thus leads to
the failure of the triads.
Dobereiner triads:
Dobereiner in 1817 observed that certain elements, which had similar chemical properties, could be grouped together. When these
elements were arranged in increasing order of their atomic masses, they generally occurred in groups of three. These groups were
called triads. He noticed that the atomic mass of the middle element of the triad was the arithmetic mean of the other two
elements of the triad. This was known as the Dobereiner's law of triads. The law states that when elements are placed in the
ascending order of atomic masses, groups of three elements having similar properties are obtained. The atomic mass of the middle
element of the triad is equal to the mean of the atomic masses of the other two elements of the triad.
Drawbacks of Dobereiner's law of triad were that, it was valid only for a few groups of elements known during that time. Also with more accurate measurements of atomic masses showed that the mid element of the triad did not really have the mean value of the sum of the other two elements of the triad.
Example of Dobereiner triads:
In the alkali metal group, consider elements lithium (Li), sodium (Na) and potassium (K). All these elements are metals; they are
highly reactive and show valency of +1. The Dobereiner's triad for alkali metal group can be shown as:
From the Dobereiner's law of triads, the atomic mass of the middle element, in this case Na, should be the
arithmetic mean of Li and K. Thus,
It can be seen that, arithmetic mean of atomic masses of Li and K = atomic mass of Na.
A hydrogen atom, having only the K shell, can achieve a maximum of two electrons, i.e., a duplet, like helium. A lithium atom can also achieve only a duplet and never an octet.
Dobereiner triads
Dobereiner's law of triads failed for the following reasons:
- All the known elements found later could not be arranged in the form of triads.
- For very low mass or for very high mass elements, the law was not holding good. Take the example of F, Cl and Br. Atomic mass of Cl is not an arithmetic mean of atomic masses of F and Br.
- As the techniques improved for measuring atomic masses accurately, the law was unable to remain strictly valid.
The only advantage of Dobereiner's research was that it made chemists look at elements in terms of groups of elements with similar chemical and physical properties. This eventually led to rigorous classification of elements and the modern periodic table of elements, as we now know it, was discovered.