Question

The atomic number of an element is more important to the chemist than its relative atomic mass. Why?

Answer 1

Henry Moseley found that when cathode rays struck anodes of different metals, the wavelength of these metals was found to decrease in a regular manner of changing the metal of anode in the order of its position in the periodic table. By this, he concluded that the number of positive charges present in the nucleus due to protons (atomic number) is the most fundamental property of the element.

So, Henry Moseley found that the atomic number is a better fundamental property of an element compared to its atomic mass. This lead to the modern periodic law.

This law gave explanations for anomalies in Mendeleev's classification of elements such as

1. The position of isotopes with the same atomic number can be put in one place in the same group.
2. Position of argon and potassium: Potassium with a higher atomic number should come later, and argon with a lower atomic number should come first.