How does atomic weight of something change material properties?

Question

Why does gold look and behave like it does and alter a proton or two (whatever it takes) and it becomes iron or oxygen?

Answer 1

It's less about atomic weight and more about atomic number. Isotopes, the same element with a different number of neutrons in its nuclei, normally behave the same as each other, except that some are less stable and eventually decay.

The main motivators for material properties are actually electrons. Electrons are always trying to fill orbitals at the most stable level possible. This governs what other elements the atoms react with and how they behave relative to each other.