Why Does Copper Turn Green?

clipped from http://www.livescience.com/mysteries/071003-llm-copper-green.html

Just as iron that is left unprotected in open air will corrode and form a
flaky orange-red outer layer, copper
that is exposed to the elements undergoes a series of chemical reactions that
give the shiny metal a pale green outer layer called a patina.

The patina actually protects the copper below the surface from further
corrosion, making it a good water-proofing material for roofs (which is why the
roofs of so many old
buildings are bright green).

In fact, the weathering and oxidation of the Statue
of Liberty's copper skin has amounted to just .005 of an inch over the last
century, according to the Copper Development Association.