What's the difference between plutonium 239 and uranium 238?

One proton, and one electron when the atom is electrically neutral. The plutonium has that one more proton than the U238, but no more neutrons than it has in the nucleus. With the extra electron in its shells, the chemical behavior of the plutonium is different from that of U238. For one thing, if I remember correctly, plutonium is one of the most deadly poisons chemically, not radioactively, that is known to man.