Gold/Au (atomic number #79#) has only one electron in its outer valence shell.
The #10 xx # #5d# electrons in gold are in a filled energy level, leaving only one electron in the outer shell. The ground state configuration of Gold is
#[Xe] 5d^10 6s^1#
The energy level difference between the #6s# and #5d# is small. This makes it possible for one of the two #6s# electrons to rather be in the #5d# orbitals.
When the #5d# has #10# electrons, the #5d# orbitals are filled. The filled #5d# orbitals make gold very stable.
Furthermore, the #6s# orbital contracts due to scalar relativistic effects (where the #6s# electrons move at about half the speed of light), becoming relatively inaccessible to many reactants, making gold quite inert.
