According to class 9 Science chapter 4 notes pdf download, atomic number is the number of protons present in the nucleus of an  element. Number of protons is the same as the number of electrons. While atomic mass number is both the number of protons and neutrons in an atom. Since most of the reactions depend on the valence electrons, atomic number is more essential. So, atomic mass number or atomic weight is less essential in determining the chemical characteristics of elements.