The elements in the periodic table are arranged in order of the increasing atomic number because elemental characteristics follow trends in the atomic number, which makes the atomic number define an element. If you were to use the mass number which are protons and neutrons some high neutrol isotopes might push an element that should be in group 7, into group 8, or low neutrons could push it into group 6, though it would still be a halogen and have the characteristics of halogens.
