Question

Why Dobereiner, Newlands and Mendeleef were not 100% successful in their classification of elements? Why is the modern table a relatively better classification? Predict the reason.

Answer 1

Hint: In the periodic table, elements are arranged according to a fundamental property. The classification is based on either the atomic number of elements or on the atomic mass. In the periodic table, several horizontal rows and vertical periods are present. The elements in a vertical group show similarities in chemical properties.

Complete answer:
> Dobereiner arranged elements having similar properties into a group of three, called triads. The atomic weight of the second element was roughly the average of the atomic weights of the first and third element. Dobereiner could arrange only nine elements in three triads. But several elements were known at that time, that cannot be arranged in Dobereiner triads
> Newland gave the law of octaves and discovered that when elements are arranged in ascending order of atomic weights, every eighth element has properties similar to the first element. He compared this to eight octaves of music. Out of 56 known elements at that time, Newland could arrange only elements upto calcium. This is because, after calcium, every eighth element did not have properties similar to the first element.
> Mendeleev observed that there exists a relationship between the atomic masses of the elements and their chemical properties. In the Mendeleev's periodic table, some elements (such as cobalt 27 and nickel 28) have anomalous positions. This is because the atomic mass of cobalt is higher than that of nickel, but the atomic number of nickel is higher than that of cobalt.
> Dobereiner, Newlands and Mendeleef were not 100% successful in their classification of elements. This is because they based their classification on the atomic mass of elements. Atomic mass is not the fundamental property of elements.
> However, the modern table is relatively a better classification as it is based on the atomic number of elements. Atomic number is the fundamental property of elements.

Additional Information: The following points illustrate why the modern periodic table is superior.
- In the modern periodic table, the position of isotopes can be justified. There is no need to give separate positions to isotopes.
- Since hydrogen resembles halogens and alkali metals, hydrogen is given a unique position in the modern periodic table.
- Modern periodic table is easy to remember and easy to reproduce. Based on the position of the elements, in the groups and periods, it is easier to predict the properties.
- In the Mendeleev's periodic table, some elements (such as cobalt 27 and nickel 28) have anomalous positions. In the modern periodic table, the positions of these elements are justified.

Note: According to modern periodic law, “The physical and chemical properties of the elements are the periodic functions of their atomic numbers”.