Question

Why is the atomic number a more fundamental than atomic mass in determining chemical properties?

Answer 1

Chemical properties depend in the no. of electrons in an atom and their arrangement in different shells.
No. of electrons is equal to no. of protons.
No. of protons is the atomic number.
Hence, atomic no. is fundamental than Mass no.
Mass no. of two elements can be sane whereas atomic number is unique for each element.
Argon and Calcium has same atomic mass (39.9 & 40.1 respectively) but have no common chemical properties.

Answer 2

The number of electrons in turn is equal to the number of protons in the nucleus. Atomic number is a more fundamental property of an element than its atomic mass because when the elements are arranged in the increasing order of their atomic number, most of the defects of Mendeleev's classification get rectified.

I hope you are understand