Early Attempts at the Classification of Elements: Dobereiner, Newlands

Early Attempts at the Classification of Elements: All matter in the universe is made up of elements, compounds, and mixtures. Elements can be found in all compounds and mixtures. Only \(30\) elements were known until the end of the year \(1800\). Elements were discovered at a rapid pace in the nineteenth and twentieth centuries, and several new elements were discovered.

At present, one hundred and eighteen elements are known to us. These elements combine to give a large number of compounds. The elements and their compounds (quite large in number) show great variation in their chemical behaviour, and it was impossible to study them haphazardly. With the discovery of more and more elements, the chemists realised that these elements should be arranged properly to facilitate their systematic study.

Several efforts were made to classify the known elements systematically. Few important attempts for the classification of elements are outlined in this article.

Early Attempts at the Classification of Elements

Lavoisier initially classified the elements as metals and nonmetals, but this classification missed some elements that could not be classified as metals or nonmetals.

Later on, such elements were named metalloids. This kind of classification was insufficient for systematic studies of all the elements. Thus, several chemists attempted to make a more rational and systematic classification of elements opening new insights into understanding the properties of the elements. Thus, periodic classification of elements evolved.

Unitary Theory

In \(1815\), William Prout suggested that hydrogen is the basis of the atomic masses of all elements. The atomic masses of all elements were whole numbers or varied only slightly from the whole numbers. Although this theory could not stand for a long time, it gave an idea for the first time that the elements were related to one another through their atomic masses.

Dobereiner’s Triads

A German chemist, Johann Wolfgang Dobereiner, made one of the earliest attempts to classify elements discovered at that time. In \(1817\), Dobereiner noticed that some elements formed groups of three with similar properties. These groups of three elements each were termed triads. Some triads classified by Dobereiner are:

Study Classification of Elements

What was the most important property of elements belonging to a Döbereiner’s Triad?

In the triads of elements, the atomic weight of the middle element was the arithmetic mean of the atomic weights of the other two. Some of the triads are as under. For example:

Triad 1: The Alkali Metal Triad

The elements lithium \({\rm{(Li)}}\), sodium \({\rm{(Na)}}\) and potassium \({\rm{(K)}}\) are all metals and react with water to form strong alkalis. Thus, these are grouped together to form a triad known as the alkali metal triad.

The atomic mass of the middle element, sodium, is the arithmetic mean of the atomic weights of lithium and potassium, as shown below.

Arithmetic mean of atomic masses of \({\rm{Li}}\) and \({\rm{K}}\) \( = \frac{{7 \cdot 0 + 39 \cdot 0}}{2} = 23.0 = {\rm{ Atomic\, mass\, of\, Na}}\)

Triad 2: The Alkaline Earth Metal Triad

The elements Calcium \({\rm{(Ca)}}\), Strontium \({\rm{(Sr)}}\) and Barium \({\rm{(Ba)}}\) are all metals. Their oxides and hydroxides are alkaline in nature, and these metal oxides are found in the earth’s crust. Thus, these are grouped together to form a triad known as an alkaline earth metal triad.

The atomic mass of the middle element, strontium, is almost the arithmetic mean of the atomic weights of the Calcium and Barium, as shown below.

Arithmetic mean of atomic masses of \({\rm{Ca}}\) and \({\rm{Ba}}\) \( = \frac{{40.0 + 137.0}}{2} = 88.5 \approx {\rm{Atomic}}{\mkern 1mu} \,{\rm{mass}}\,{\mkern 1mu} {\rm{of}}\,{\rm{Sr}}(88.0)\)

Triad 3: The Halogen Triad

The elements chlorine \({\rm{(Cl)}}\), bromine \({\rm{(Br)}}\) and iodine \({\rm{(I)}}\) are all non-metals and salt-forming elements. Thus, these are grouped together to form a triad known as the halogen triad.

The atomic mass of the middle element, bromine, is almost the arithmetic mean of the atomic weights of the chlorine and iodine, as shown below.

Arithmetic mean of atomic masses of \({\rm{Cl}}\) and \({\rm{I}}\) \(= \frac{{35.5 + 127.0}}{2} = 81.25 \approx {\rm{Atomic}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{Br}}\,(80.0)\).

Limitation of Döbereiner’s Triads

The concept of triads could be applied only for the limited number of elements. Döbereiner could find only three such triads, and he could not even put all the elements known at that time in his triads. Hence, this system of classification into triads given by Döbereiner was not found to be very useful.

Newland’s Law of Octaves

John A. R. Newlands \((1865)\), an English chemist, classified elements on the basis of Law of Octaves, like the octaves found in music (sa, re, ga, ma, pa, da, ni). That is why Newland’s law was named as Law of Octaves. According to him.

If the elements are arranged in the increasing order of their atomic masses, the properties of every eight elements are similar to the first element’s properties.

At the time of Newland, only \(56\) elements were known. He tried to arrange these elements in the form of octaves. He placed hydrogen first (lowest atomic mass) and finished at thorium, the \({\rm{56th}}\) element known at that time. On arranging elements in this fashion, he found that every eight elements resembled the first one in its properties in many cases.

Limitations of Newland’s Law of Octaves

• (a) Up to the element calcium, i.e., lighter elements, Newland’s law of octaves was applicable. After that, every eighth element did not possess similar properties as that of the first element. We shall discuss it with few examples-
• Example 1: If we start with Chromium \(({\rm{Cr}})\), then the eighth element from it will be Yttrium \(({\rm{Y}})\). These two elements have entirely different properties.
• Example 2: If we start with Titanium \(({\rm{Ti}})\), then the eighth element from it will be Indium \(({\rm{In}})\). These two elements have entirely different properties.

• (b) Newland placed two elements in the same slot and placed unlike elements in the same column to fit all \(56\) elements into his table.
• Example 1: Cobalt \(({\rm{Co}})\) and Nickel \(({\rm{Ni}})\) were placed in the same slot as that of Fluorine \(({\rm{F}})\), Chlorine \(({\rm{Cl}})\) and Bromine \(({\rm{Br}})\) which have different properties.
• Example 2: Iron \(({\rm{Fe}})\) which resembles the properties of Cobalt and Nickel, were placed under Oxygen.
• Example 3: Cerium \(({\rm{Ce}})\) and Lanthanum \(({\rm{La}})\) were placed in the same slot as that of Chromium \(({\rm{Cr}})\) and Yttrium \(({\rm{Y}})\) though their properties are different from each other.
• Thus, Newland failed to explain such a position of the elements.

• (c) Newland predicted that only \(56\) elements would exist in nature and that no new elements would be discovered in the future. However, several new elements were discovered later whose properties contradicted the law of octaves.

• (d) When Newland gave his law of octaves, noble gas elements were not discovered. However, when noble gases were discovered, the properties of the first element and eighth element were not similar, and they became the first and ninth elements. Thereby, the entire arrangement was disturbed. Thus, Newlands’ law of octaves was also discarded.

Lothar Meyer’s Graph

Lothar Meyer \((1869)\) plotted a graph of atomic volume vs atomic mass of different elements. Similar elements were found to occupy similar positions, e.g., alkali metals occur on peaks of graphs.

Lavoisier’s Classification

Lavoisier categorises elements as metals or nonmetals. This classification was based on physical characteristics such as hardness, malleability, and lustre. Based on these characteristics, sodium and lead were classified as metals.

Limitations of Lavoisier’s Classification

1. The only properties that sodium and lead had in common were hardness, malleability, and lustre. Otherwise, the two elements were diametrically opposed.
2. There was no room in such a classification for elements with properties similar to metals as well as non-metals.
3. Therefore, Lavoisier’s classification was found to be inadequate.

Summary

All of these elements in the periodic table were not discovered in a day. When very few elements were known, studying them separately was not a problem. But when a large number of elements had been discovered, it became difficult to study the properties of all of them separately.

So, from time to time, attempts were made to sort out the elements into groups to follow their behaviour in orderly. Studying the properties of a typical element of a particular group enables scientists to roughly predict other elements properties. We have discussed New lands classification, Dobereiners classification, Lothar Meyer curves, Lavoisier’s classification in this article.

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FAQs on Early Attempts at Classification of Elements

Q.1. What were the criteria used for the early attempts of classification?
Ans: In \(1789\), Antoine Lavoisier attempted to categorise the elements by grouping them into gases, non-metals, and metals based on their characteristics. Several more attempts to put components together were attempted during the next few decades.

Q.2. What is the classification of elements?
Ans: The classification of elements became necessary with the discovery of new elements. Different scientists attempted to look for some trends and patterns in the properties of then known elements based on which the study of such a large number of elements could become easy.

Q.3. What are the early attempts to classify elements were based?
Ans: Early chemists sought to categorise elements based on their atomic weights or relative atomic mass as it is now known. The relative atomic masses of the elements grew in steps, they could observe. They also had flaws, such as components that were not related to being grouped together.

Q.4. What was the reason for the classification of elements?
Ans: The grouping of elements creates a fixed pattern in which the characteristics of the elements vary periodically. The periodic table simplified and structured the study of the physical and chemical characteristics of elements.

Q.5. State Newland’s law of Octaves.
Ans: When the elements with lower atomic masses were arranged in order of their increasing atomic masses, the properties of every eighth element were similar to those of the first one, like the eighth note of a musical scale.

Q.6. The modern periodic table has been evolved through the early attempts of Dobereiner, Newland and Mendeleev. List one advantage and one limitation of all three attempts.
Ans: Attempt of Dobereiner
Advantage: Dobereiner could predict the atomic mass of a middle element in each triad.
Limitation: Dobereiner could identify only three triads.
Attempt of Newland
Advantage: Every eighth element had properties similar to that of the first. Thus, Newland was able to co-relate the properties of elements with their atomic mass,
Limitation: It was only applicable up to calcium.
Attempt of Mendeleev
Advantage: Mendeleev grouped the elements with similar properties together. He could also predict the existence of new elements that had not been discovered at that time. Limitation: The position of isotopes was not justified.

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