Which of the following chemical formulas represents both an element and a molecule?

• A. O2
• B. NaCl
• C. CO2
• D. H2O

Answer: (A)

Verified step by step guidance

1. Understand the difference between an element and a molecule: An element consists of only one type of atom, while a molecule is formed when two or more atoms are chemically bonded together. A molecule can be made of the same type of atoms (elemental molecule) or different types of atoms (compound).
2. Analyze each chemical formula to determine if it represents an element or a compound: For example, NaCl contains sodium (Na) and chlorine (Cl), so it is a compound, not a pure element.
3. Check if the formula consists of only one type of atom bonded together, which would make it both an element and a molecule. For example, O2 consists of two oxygen atoms bonded together, so it is a molecule made of a single element.
4. Identify that CO2 and H2O are compounds because they contain more than one type of atom (carbon and oxygen in CO2, hydrogen and oxygen in H2O), so they are molecules but not elements.
5. Conclude that O2 is the correct answer because it represents a molecule composed of only one element, oxygen, making it both an element and a molecule.