Which of the following substances can copper (Cu) react with under standard conditions to form a compound?

• A. Argon (Ar)
• B. Helium (He)
• C. Oxygen (O_2)
• D. Nitrogen (N_2)

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the reactivity of copper (Cu) under standard conditions. Copper is a transition metal that can form compounds by reacting with certain elements, especially nonmetals like oxygen.
2. Step 2: Analyze the given substances: Argon (Ar) and Helium (He) are noble gases, which are very stable and generally do not react with metals under standard conditions due to their full valence electron shells.
3. Step 3: Consider Nitrogen (N_2), which is a very stable diatomic molecule with a strong triple bond. Copper does not typically react with nitrogen under standard conditions to form stable compounds.
4. Step 4: Look at Oxygen (O_2), which is a reactive diatomic molecule. Copper readily reacts with oxygen to form copper oxides (such as CuO or Cu_2O) under standard conditions.
5. Step 5: Conclude that among the given options, copper can react with oxygen (O_2) to form compounds under standard conditions, but not with argon, helium, or nitrogen.