More is different
What’s the difference between neon and ammonia?
In its most common isotope, a single neon molecule (one atom, in fact) contains 10 neutrons, 10 electrons, and 10 protons. A single ammonia molecule also contains 10 neutrons, 10 electrons, and 10 protons. It’s the same stuff! I think this is very cool.
Update: Well, I can’t count. Ammonia only has 7 neutrons. I know this kind of phenomenon is possible, because I set it as a problem once in a mini-course I gave on metals and superconductors, and people came back with several solutions. Unfortunately, I don’t remember what they were.
Update II: Potassium Bromide (K Br) and Calcium Selenide (Ca Se) appear to do the trick, assuming no more silly mistakes. Can anyone find a simpler example?
Update III: Helium and Deuterium both have 2 protons, 2 neutrons, and 2 electrons. It’d still be nice to have examples involving two more familiar substances.
Update IV: Commenter Kurt points out a better example: Nitrous Oxide (laughing gas) and Carbon Dioxide, both with 22 electrons, protons, and neutrons. Any better? I think salt and nickel 58 (the most common isotope) also provide an example.