Which of the following is an atom rather than a molecule?

• A. H2O (water molecule)
• B. O (oxygen atom)
• C. CO2 (carbon dioxide molecule)
• D. O2 (oxygen molecule)

Answer: (B)

Verified step by step guidance

1. Understand the difference between an atom and a molecule: An atom is a single unit of an element, while a molecule consists of two or more atoms chemically bonded together.
2. Examine each option: H2O is water, which consists of two hydrogen atoms and one oxygen atom bonded together, so it is a molecule.
3. CO2 is carbon dioxide, made up of one carbon atom and two oxygen atoms bonded together, so it is also a molecule.
4. O2 is oxygen gas, composed of two oxygen atoms bonded together, making it a molecule as well.
5. O (oxygen atom) is a single oxygen atom, not bonded to any other atom, so it is an atom rather than a molecule.