Which of the following correctly identifies an element?

• A. Water (H2O) is an element.
• B. Air is a compound.
• C. Soil is a mixture.
• D. Oxygen (O2) is an element.

Answer: (D)

An element is a pure substance made of only one type of atom and cannot be broken down into simpler substances by chemical means. Oxygen gas fits this because it consists solely of oxygen atoms, even though they pair up to form O2; diatomic molecules are still the same element. Water, H2O, is a compound formed when hydrogen and oxygen atoms bond in a fixed ratio, giving it properties different from either element alone. Air is a mixture of several gases, mainly nitrogen and oxygen plus others, so it isn’t a single substance. Soil is a mixture of minerals, organic matter, air, and water, not an element.