The reason of why some metals lose their shine over time but the gold does not is so simple that's because the corrosion and tarnish processes. Metals donate electrons when exposed to atmosphere and tends to form oxide. and when the metal donate electrons the oxygen tends to accept this electrons and fill up its shell and form oxide and the metal lose its shine.when the oxide film contact with an aqueous medium metal oxide pairs with H2O and form rust.
But gold is a noble metal (like platinum and silver), it remains stable in its pure form and doesn't donate electrons so, no loss electrons and the oxide don't happen at all. So the gold is known for its resistance to oxidation. As the noble metals have a very low reaction rate with oxygen, So there is no rusting as the rusting is the result of the reaction of the oxygen with the metals.So there is no tarnish appears. that's why the gold doesn't lose its shine and keeps its pure form.
