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.
