Early Attempts – Classification of Elements

Early Attempts – Classification of Elements

Early Attempts - Classification of Elements

In early 19th Century, many scientists made number of attempts to arrange and classify elements based on their elemental properties. Some of these classification are important:

1. Johann Dobereiner Triad Model

2. Newland’s Law Of Octaves


Before going in for discussion on these model’s, It’s better to have basic understanding of few concepts & terms used in these models.

Johann Dobereiner Triad Model

 

John Dobereiner was a chemist & scientist in early 19th century who tried to solve the mystery of arranging all the known (at that time) elements in a tabular manner.

He proposes to arrange elements in a Triad Model, with following features:

  • He tried to arrange elements in groups of three called Triads
  • Elements with similar properties (both Physical and Chemical) are placed in the same group
  • Elements were arranged in the increasing order of their atomic mass
  • Atomic mass of the middle element was the average of the other two elements

Example:

  • The elements (x,y,z) were arranged in order of increasing Atomic Mass
  • The atomic mass of the middle element (y) was almost the mean or average of the atomic mass (A) of the first and third elements i.e. x and z
  • A of element (y) = ½ [A of element (x) + A of element (z)]

Triads of (Li, Na  & K), (Cl, Br & I), (Ca, Sr & Ba)

Johann Dobereiner Triad Model

Limitations of Dobereiner’s Triads

  • Triad representation was not feasible for all the elements known at that time
  • Only three triads (Li, Na  & K), (Cl, Br & I), (Ca, Sr & Ba) based on Atomic Mass were identified

Newland’s Law Of Octaves

 

John Newland was also a chemist & scientist in early 19th century who tried to solve the mystery of arranging all the known (at that time) elements in a tabular manner. At the time of Newland 56 elements were known.

Following are the properties for Newland’s element arrangement:

  • Elements were arranged on the basis of their atomic mass
  • Newland follow the Law of Octaves for the arrangement of elements in periodic order
  • Law of Octaves
    • It symbolizes for the repetition of element behavior and their properties for every eighth element in the series
    • It was similar to repetition of musical notes in Indian & Western musical system
    • Indian system – sa, re, ga, ma, pa da, ni, sa
    • Western system – do, re, me, fa, so, la, ti, do
Newland's Arrangement of Elements
Newland's Arrangement of Elements

Newland’s periodic arrangement:

  • Vertical rows are known as Groups
  • Elements placed in groups have common properties

Example:

  • Elements in (ga) group – Be, Mg and Ca have similar elemental properties

Limitations of Newland’s Law of Octave:

  • After Chlorine (Cl) element, other elements could not be accommodated into octaves
    • Like, Cobalt (Co) and Nickel (Ni) placed in the same group as Chlorine, but have different properties as compared to other elements in the same group
  • At many places, two elements were places in the same slot
    • Like, Cobalt (Co) & Nickel (Ni) and Cerium (Ce) & Lanthanum (La) were placed in same slot
  • No places were left for undiscovered elements
    • At Newland’s time, many elements were not discovered and unknown like, Noble gases [Helium (He), Neon (Ne), Argon (Ar)]
    • Arranging them at later stage, was very difficult and disturbed the entire arrangement

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