Why does copper turn green when exposed to air? 

Due to the formation of copper carbonate.

The correct answer is option 2. Due to the formation of copper carbonate.

Copper turns green when exposed to air primarily due to the formation of copper carbonate.

When copper is exposed to air, it reacts with carbon dioxide \((CO_2)\) and moisture (water vapor) in the air to form a thin layer of copper carbonate \((CuCO_3)\) on the surface of the metal. This greenish layer is known as patina or verdigris. The chemical reaction can be summarized as follows

\[ \text{2Cu} (s) + \text{O}_2 (g) + \text{CO}_2 (g) + \text{H}_2\text{O} (l) \rightarrow \text{CuCO}_3 \text{(s)} \]

In this reaction, copper reacts with oxygen (from air), carbon dioxide (from air), and water vapor (from air) to form copper carbonate. This green copper carbonate layer acts as a protective barrier, preventing further corrosion of the underlying copper metal. Over time, this layer may continue to develop and darken, giving old copper surfaces their characteristic greenish hue.

While copper oxide \((Cu_2O)\) can also form on the surface of copper, especially at higher temperatures, the greenish color observed on weathered copper surfaces is primarily due to the presence of copper carbonate. Copper sulfide or copper hydride do not contribute significantly to the green color observed on weathered copper surfaces.