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)
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 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
References:
- Learning Course for full lifetime access – Periodic Classification of Elements
- Learning Videos – Periodic Classification of Elements
- Modern Periodic Table
- Mendeleev’s Periodic Table
- Periods & Groups
- Periods in Detail
- Groups in Detail