Which of the following is an element?

• A. Oxygen (O)
• B. Carbon dioxide (CO2)
• C. Sodium chloride (NaCl)
• D. Water (H2O)

Answer: (A)

Verified step by step guidance

1. Understand the definition of an element: An element is a pure substance consisting of only one type of atom, represented by a unique chemical symbol on the periodic table.
2. Analyze each option to determine if it is an element or a compound: Elements have single symbols (like O for oxygen), while compounds are made of two or more elements chemically bonded together.
3. Look at Oxygen (O): It is represented by a single chemical symbol and consists of only oxygen atoms, so it is an element.
4. Look at Carbon dioxide (CO2): It contains carbon and oxygen atoms chemically bonded, so it is a compound, not an element.
5. Look at Sodium chloride (NaCl) and Water (H2O): Both are compounds made of more than one element chemically combined, so they are not elements.