Gold stays shiny because it barely wants to bond with oxygen
Gold pulled from a riverbed after thousands of years still gleams, because it has almost no appetite for oxygen; its oxides are so unstable they fall apart on their own. That aloofness defines the noble metals, and it explains why people have trusted them for jewellery and coins for millennia.
The core members are gold plus platinum and its five chemical cousins, iridium, osmium, palladium, rhodium and ruthenium. Silver, copper and mercury sometimes join the club, though in nature they are usually locked up with sulphur. Along with copper and silver, gold and the platinum metals are the only elements found as pure native metal in sizeable amounts. The label is old, reaching back at least to the late fourteenth century, and a chemist named Odling grouped the platinum metals beside silver and gold in a table of 1864, five years before Mendeleev's famous periodic table.
Nobility has a physical basis. These metals are fairly electronegative, holding their electrons tightly, so any bond they form with oxygen is weak. A metallurgist in 1946 offered a practical test: a noble metal is one whose oxide breaks down at temperatures below red heat. Compare sodium or potassium, which grab oxygen so eagerly that they ignite in air. Gold is electronegative enough to form a negatively charged ion, found in a yellow salt with caesium.
Noble does not mean indestructible. Aqua regia, a fierce blend of hydrochloric and nitric acids, dissolves gold and platinum, and in 2010 researchers found a milder organic mixture that could be tuned to dissolve gold while leaving palladium and platinum alone. Silver is the awkward case. It blackens in sulphur-bearing air, and one chemist argues it is too reactive to deserve the title; dentists do not count it as noble because it corrodes in the mouth.
These metals also love iron. They dissolve so readily in it that most of Earth's supply probably sank into the planet's core as it formed. What remains near the surface works hard: all the noble metals are useful catalysts, and platinum in car catalytic converters turns toxic nitrogen oxides into harmless gases. Chemists predict that several superheavy elements, from hassium onward, should behave as noble metals too.
Source: Noble metal
