However, gold is not as inert as one would like to think. There is thus an entire series of compounds that contain gold atoms in various oxidation states. In these compounds, the gold can exist as either a positive or, as has been known for a few years, a negative ion.
Electron affinity is a measure of how strongly an atom can bind an additional electron.
Thus positively and negatively charged gold particles are formed from elemental gold, a process called disproportionation. This group is called aurate. Beside these, in the crystal are Au ions with a single negative charge, known as auride ions. These gold compounds are amazingly stable.