Class 10 Chemistry
Early attempts of classification of elements:
Johann Wolfgang Dobereiner was a German chemist. His effort is considered as one of the earliest attempts to classify the elements into groups.
He found that when elements are arranged into groups of three in the order of their increasing atomic mass, the atomic mass of the element; which comes in the middle; is the arithmetic mean of rest of the two. On this basis, he arranged three elements in one group which is known as ‘Triad’. This arrangement of elements is known as Dobereiner’s Triads.
|Elements and their Atomic Mass
|Lithium (Li) 7.0
||Sodium (Na) 23.0
||Potassium (K) 39.0
|Calcium (Ca) 40.0
||Strontium (Sr) 87.5
||Barium (Ba) 137.0
|Chlorine (Cl) 35.0
||Bromine (Br) 80.0
||Iodine (I) 127.0
In this table, atomic mass of sodium is equal to arithmetic mean of atomic masses of lihtium and potassium. Similarly, atomic mass of strontium is equal to arithmetic mean of atomic masses of calcium and braium.
Limitation of Dobereiner’s Triads:
Dobereiner could find only three such triads (group of three elements) and he could not even put all the elements known at that time in his triads.
The rules of Dobereiner’s triads could not be applied to the elements which had very low or high atomic mass. Such as; if F, Cl and Br are put together in a triad, in increasing order of their atomic masses, the atomic mass of Cl is not an arithmetic mean of atomic masses of F and Br.
After the advancement of techniques of measuring atomic mass more correctly Dobereiner’s Law became obsolete.
Newlands’ Law of Octaves:
Newlands found that every eighth element has similar physical and chemical properties when they are arranged in increasing order of their relative masses. This law is known as Newlands’ Law of Octaves which states that "any given element will exhibit analoogus behaviour to the eighth element following it in the table". This means every eight element has the similar chemical and physical properties. For example; Sodium is the eighth element from Lithium and both have similar properties.
The arrangement of elements in Newlands’ Octave resembles the musical notes. In musical notes, every eighth note produces similar sound. Because of this; Newland’s classification of elements was popularly known as just Octaves.
Limitation of Newlands’ Octaves:
- Newlands’ Octaves could be valid upto calcium only; as beyond calcium, elements do not obey the rules of Octaves.
- Newlands’ Octaves was valid for lighter elements only.
- It appears that Newlands did not expect the discovery of more elements than 56 which were discovered till his time.
- More than one element had to be placed in some of the groups; in order to place the elements having similar properties in one group. But in order to do so, he also put some dissimilar elements in same group.
- Iron; which has similar property as cobalt and nickel, was placed far from them.
- Cobalt and nickel were placed in the group with chlorine and fluorine in spite of having different properties.
- In spite of above limitations; Newlands was the first scientist who arranged the elements in order of their increasing relative atomic masses.
Modern Periodic Table