Dobereiner’s triads and Newland’s law of octaves were early attempts at classifying elements into groups based on their properties. Since many new elements were discovered over the course of the 18th and 19th centuries, the broad classification of elements into metals and non-metals became inefficient. Several experiments were conducted in order to identify elements with similar properties and group them together.
It is important to note that the primitive methods of classifying elements, such as Newland’s law of octaves and Dobereiner’s triads, laid the foundation for the development of the modern periodic table.
What are Dobereiner’s Triads?
Dobereiner’s triads were groups of elements with similar properties that were identified by the German chemist Johann Wolfgang Dobereiner. He observed that groups of three elements (triads) could be formed in which all the elements shared similar physical and chemical properties.
Dobereiner stated in his law of triads that the arithmetic mean of the atomic masses of the first and third element in a triad would be approximately equal to the atomic mass of the second element in that triad. He also suggested that this law could be extended for other quantifiable properties of elements, such as density.
The first of Dobereiner’s triads was identified in the year 1817 and was constituted by the alkaline earth metals calcium, strontium, and barium. Three more triads were identified by the year 1829. These triads are tabulated below.
This triad was made up of the alkali metals lithium, sodium, and potassium.
The arithmetic mean of the masses of potassium and lithium corresponds to 23.02, which is almost equal to the atomic mass of sodium.
As mentioned earlier, calcium, barium, and strontium formed another one of Dobereiner’s triads.
The mean of the masses of barium and calcium corresponds to 88.7.
The halogens chlorine, bromine, and iodine constituted one of the triads.
The mean value of the atomic masses of chlorine and iodine is 81.1.
The fourth triad was formed by the elements sulfur, selenium, and tellurium.
The arithmetic mean of the masses of the first and third elements in this triad corresponds to 79.85.
Iron, cobalt, and nickel constituted the last of Dobereiner’s triads.
However, the mean of the atomic masses of iron and nickel corresponds to 57.3.
Limitations of Dobereiner’s Triads
The key shortcomings of Dobereiner’s method of classifying elements are listed below.
- The identification of new elements made this model obsolete.
- Newly discovered elements did not fit into the triads.
- Only a total of 5 Dobereiner’s triads were identified.
- Even several known elements did not fit into any of the triads.
Owing to these shortcomings, other methods of classifying elements were developed.
Newland’s Law of Octaves
In the year 1864, the British chemist John Newlands attempted the 62 elements known at that time. He arranged them in an ascending order based on their atomic masses and observed that every 8th element had similar properties. On the basis of this observation, Newland’s law of octaves was formulated.
The law of octaves states that every eighth element has similar properties when the elements are arranged in the increasing order of their atomic masses. An illustration detailing the elements holding similar properties as per Newland’s law of octaves is provided below.
Newlands compared the similarity between the elements to the octaves of music, where every eighth note is comparable to the first. This was the first attempt at assigning an atomic number to each element. However, this method of classifying elements was met with a lot of resistance in the scientific community.
Limitations of Newland’s Law of Octaves
The key shortcomings of Newland’s law of octaves are listed below.
- Several elements were fit into the same slots in Newland’s periodic classification. For example, cobalt and nickel were placed in the same slot.
- Elements with dissimilar properties were grouped together. For example, the halogens were grouped with some metals such as cobalt, nickel, and platinum.
- Newland’s law of octaves held true only for elements up to calcium. Elements with greater atomic masses could not be accommodated into octaves.
- The elements that were discovered later could not be fit into the octave pattern. Therefore, this method of classifying elements did not leave any room for the discovery of new elements.
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