The sequence of the periodic table based solely on atomic mass was initially proposed by Dmitri Mendeleev in the 19th century. While it provided a useful framework for organizing the known elements at that time, it was later refined and modified based on additional experimental data and a deeper understanding of atomic structure. The rejection of a strict atomic mass-based sequence can be attributed to the following reasons:
Anomalies in chemical properties: When elements were arranged solely by atomic mass, some elements did not fit into their expected positions based on their chemical properties. For example, iodine (I) with a higher atomic mass was placed before tellurium (Te) with a lower atomic mass, even though their chemical properties suggested the opposite order. The discovery of isotopes helped explain these discrepancies.
Isotopes: Isotopes are different forms of an element that have the same number of protons but different numbers of neutrons. Isotopes were discovered after Mendeleev's periodic table was proposed. Since isotopes have different atomic masses, a strict ordering of elements based solely on atomic mass would not accurately reflect their chemical behavior.
Periodic Law and electron configuration: The modern understanding of the periodic table is based on the concept of the periodic law, which states that the chemical and physical properties of elements are periodic functions of their atomic number (number of protons). The arrangement of elements in the periodic table is now based on the electron configuration, which determines an element's chemical behavior. This electron configuration is primarily determined by the number of protons in the nucleus (atomic number), rather than the atomic mass.
Prediction of undiscovered elements: The revised periodic table based on atomic number allows for the prediction of properties and the existence of undiscovered elements. This was not possible with the atomic mass-based table.
Ultimately, the rejection of a strict atomic mass-based sequence in favor of the modern periodic table based on atomic number and electron configuration has provided a more accurate representation of the elements and their properties.