Why is gold so chemically unreactive?

The answer

Gold's remarkable resistance to corrosion is explained by relativistic effects. Gold's inner electrons orbit so fast (about 58% of the speed of light) that relativistic mass increase causes the outer electron shell to contract, holding electrons more tightly and making them reluctant to participate in chemical reactions. This is also why gold has its distinctive yellow color rather than being silver-colored like most metals. Without Einstein's relativity, gold would behave much more like silver chemically.