Copper has a beautiful reddish hue, but when exposed to the elements, the metal undergoes a series of chemical reactions that make it turn green. The cause behind this phenomenon is similar to why iron rusts; if iron is left unprotected in open air, it will corrode and form a flaky orange-red outer layer. When copper metal corrodes, it forms what is called an oxide layer. The oxide layer forms when the surface of copper reacts with the oxygen and water present in Earth's atmosphere. The layer is made up of copper salts and oxygen, and becomes thicker over time. Eventually, the copper underneath the layer is no longer exposed enough to the air to react.

Source: Live Science