Dobereiner's Triads

In the famous atomic theory of John Dalton (1805), it was suggested that the atoms of an element have a characteristic mass. So, attempts were made to classify elements on the basis of their atomic masses.

In the year 1829, Johann Wolfgang Dobereiner, a German scientist, was the first to classify elements into groups based on John Dalton's assertions. He grouped the elements with similar chemical properties into clusters of three called 'Triads'. The distinctive feature of a triad was the atomic mass of the middle element. When elements were arranged in order of their increasing atomic mass, the atomic mass of the middle element was approximately the arithmetic mean of the other two elements of the triad.

Examples of Dobereiner's Triads

Element	Lithium	Beryllium	Potassium	Arithmetic mean
Atomic mass	7.0	9.0	11.0	9.0
Element	Lithium	Beryllium	Potassium	Arithmetic mean
Atomic mass	7.0	9.0	11.0	9.0
Element	Lithium	Beryllium	Potassium	Arithmetic mean
Atomic mass	7.0	23.0	39.0	23.0
Element	Carbon	Nitrogen	Oxygen	Arithmetic mean
Atomic mass	12.0	14.0	16.0	14.0
Element	Calcium	Strontium	Barium	Arithmetic mean
Atomic mass	40.0	87.5	137	88.1
Element	Chlorine	Bromine	Iodine	Arithmetic mean
Atomic mass	35.0	80.0	127.0	80.6

Defects of Triad Classification

A large number of similar elements could not be grouped into triads e.g., iron, manganese, nickel, cobalt, zinc and copper are similar elements but could not be placed in the triads.

It was possible that quite dissimilar elements could be grouped into triads.

Dobereiner could only classify 3 triads successfully (highlighted in the table).

Since he failed to arrange the then known elements in the form of triads his attempt at classification was not very successful.

Law of Octaves

At an earlier stage, when elements were discovered, there were very less number of elements, and hence there was no need to classify them. But as time passed, the number of elements increased and it became difficult to explain the physical and chemical properties and relations between them.

Hence, scientists developed various classifications at different times and arranged all the known elements in certain forms. First Prout gave the hypothesis that all elements are made up of hydrogen because the atomic mass of all elements is a simple integral of the atomic mass of hydrogen. But this hypothesis was not valid for a long time as some elements were discovered with fractional atomic mass.

After Prout’s hypothesis, Dobereiner gave the concept of triad in 1829. According to Dobereiner, elements can be arranged in increasing order of their atomic mass. He arranged all the known elements in groups of three each. These groups are calledtriads. In each triad, elements are arranged in increasing order of their atomic mass.The atomic mass of the middle element is equal to the average atomic mass of both the terminal elements. These triads are also termed as Dobereiner triads.

Some examples of the Dobereiner triad are as follows,

Elements with atomic weight		Average atomic weight of elements
Li₇	Na₂₃	K₃₉	7+392 =23
Cl_35.5	Br₉₀	I₁₂₇	35.5+1272 = 81.25
Ca₄₀	Sr₈₈	Ba₁₃₇	40+1372 = 88.5
S₃₂	Se₇₉	Te₁₂₇	32+1272 = 79.5
P₃₁	As₇₅	Sb₁₂₀	31+1202 = 75.5

Newlands Law of Octaves

After Dobereiner triad, John Newland purposed another way of classification in 1864.

"According to the law of octave, when elements are placed in increasing order of their atomic mass, every eighth element shows similarity with the preceding eighth element in their physical and chemical properties."

Define Octave

Octave can be defined as a group or series of eight or a series of eight notes occupying the interval between two notes, one having twice or half the frequency of vibration of the other. For example, in the series of eight musical notes, after a certain interval the note will repeat itself.

Newland's law of octaves was also based on musical notes. He arranged all the known elements in increasing order of their atomic mass. He found that in this arrangement, the first and eighth elements are similar in their chemical and physical properties. Hence there is a periodicity in elements when they are arranged in increasing order of their atomic masses.

For example, if we start from Lithium (atomic mass = 6) and arrange elements in increasing order of their atomic masses, the eighth element will be sodium (atomic mass = 23). Both elements show the same physical and chemical properties. Today, we known that both are alkali metals, located in group-1, and hence show same properties.

John Newland

John Alexander Reina Newland discovered the Periodic Table and arranged all known elements in a tabular form on the basis of their atomic weight. John Newland was a British citizen, born in London. His father was a Scottish Presbyterian minister and his mother was Italian.

His initial schooling was done at home by his father. He went to the Royal College of Chemistry for further studies but he was more interested in social reform. Hence he served as a volunteer with Giuseppe Garibaldi in 1860 to unify Italy. After that he started a laboratory in London to practice as an analytical chemist in 1864 and later he became chief chemist in James Duncan's London sugar refinery. In this refinery, he introduced a number of improvements in processing.

Newlands Periodic Table

Newland arranged all the known elements in the form of a table. There are some vertical rows, later termed as periods, and vertical columns which were later termed as groups. Newland's periodic table is shown below:

H	F	Cl	Co/Ni	Br	Pd	I	Pt/Ir
Li	Na	K	Cu	Rb	Ag	Cs	Tl
GI	Mg	Ca	Zn	Sr	Cd	Ba/V	Pb
Bo	Al	Cr	Y	Ce/La	U	Ta	Th
C	Si	Ti	In	Zr	Sn	W	Hg
N	P	Mn	As	Di/Mo	Sb	Nb	Bi
O	S	Fe	Se	Ro/Ru	Te	Au	Os

In Newland's periodic table, each row of elements consists of a total of seven elements and the eighth element falls under the first element. In the first column Lithium is the first element and sodium is the eighth element. Similarly, the eighth element after sodium is potassium. Hence from Newland's law of octaves, the elements Li, Na and K must have similar chemical and physical properties, and they do.
In the second column, Beryllium (Be) is the first element and Magnesium (Mg) is the eighth one. After magnesium the next eighth elements is calcium (Ca). Hence according to Newland's law, these elements should show similar chemical and physical properties.

In another vertical column with carbon (C) as the first element, the eighth element is silicon (Si). Both C and Si also show similar properties. Both are non-metals and show tetravalency. Thus Newland's law of octaves holds good for all these given columns. Similarly, the last group of halogens, from fluorine (F) to chlorine (Cl) also show similarity in their properties.

Limitations of Newlands Law of Octaves

This classification was good with a few elements only, and was not applicable to all elements. It was not valid for elements having atomic masses higher than Ca.
After the discovery of Nobel gases, it became difficult to fit them in Newland’s periodic table.
Newland’s octave law gave a very important term of periodicity which was repeated in further classifications.
His classification was based on the concept that the atomic mass of an element is the periodic function of it's chemical and physical properties.
The same concept of periodicity of elements was repeated in Moseley’s classification and the latest long form of periodic table is also based on the same concept. The only change is the use of atomic number instead of atomic mass.
Newland’s octave law was good till 1913. But in 1913 Henry Moseley established a new concept that the properties of elements varied periodically according to atomic number, not atomic weight.

But this concept was not applicable to elements. Only a few elements could be arranged in a given triad and some elements were not following the given criteria of atomic mass. In some triads, elements have almost same atomic weight like Fe, Co, Ni and Ru , Rh , Pd.

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Sunday 11 October 2015

EARLY CLASSIFICATION OF ELEMENTS