How copper can corrode away when it is exposed to atmosphere

it can be corrode when oxygen reacts with the atmosphere as there as some water vapors present in the air

copper and oxygen react together to make copper oxide. Since there is oxygen in the atmosphere (O2) it reacts when exposed to air.

Copper turns green because of chemical reactions with the elements. (Just as iron that is left unprotected in open air will corrode and form a flaky orange-red outer layer.)