Question

Define the Modern Periodic Law. Why was it necessary to change the basis of classification from the atomic mass to the atomic number?

Answer 1

Hint: To solve this question, you need knowledge in the history of periodic tables. Dmitri Mendeleev and Lothar Meyer gave the Modern Periodic Law.

Complete step by step answer:
We can start this answer with the definition of Modern Periodic Law. The Modern Periodic Law can be stated as: “The physical and chemical properties of the elements are periodic functions of their atomic numbers”.
It was necessary to change the basis of classification of elements from atomic masses to the atomic number. Because, the properties of elements depend upon the number of electrons present in the valence shell which are related to atomic number. Thus the properties of different elements can be compared if we know their atomic numbers. On the other hand, atomic mass can in no way determine the chemical properties of elements, because it does not vary regularly with gradation in the chemical properties of elements.

Additional Information:
The current form of periodic table widely used everywhere is the long form of the periodic table. In the long form of periodic table, the horizontal rows are called periods and vertical columns are called the groups.
Periodic law is considered as one of the Chemistry’s most important concepts. While dealing with the chemical elements, their properties, and their chemical reactions, each chemist makes use of periodic law. The development of periodic tables was given by periodic law.

Note: Dobereiner’s triads and Newlands law of octaves were early attempts at classifying elements into groups based on their properties. Since many new elements were discovered over the time of eighteenth and nineteenth centuries, the broad classification of elements into metals and nonmetals become inefficient. Many experiments and studies are conducted to identify elements with similar properties and group them together.