What makes Gold so different from Platinum?
That one electron is making sure that Gold has a complete d-orbital, and therefor is much more stable. Elements want to achieve noble gas configurations, and if they can't, then they want to have a half-filled orbitals. In the case of gold the d-orbital is filled completely and the s-orbital is half filled. Whereas Platinum doesn't have that complete d-orbital filled.

Chemical properties (such as reactivity) are influenced by electrons. The amount of neutrons will determine if it can be radioactive or not(,...) look at Tritium : that is a hydrogen atom with 2 neutrons in it and it is radioactive. Whilst protium (the most common isotope of Hydrogen with 1 proton and 0 neutrons) is not radioactive.