Question

The atomic number of an Element is more Important to the chemist than Its relative atomic Mass. Why?

Answer 1

the modern periodic law states that the properties of an element are the periodic function of its atomic number.
moreover isotopes of an element have differing atomic masses but same atomic number. however their properties are the same. thus we can see that each element can have different atomic masses but can only have one atomic number.
therefore atomic number is more important than atomic mass.