A pure substance made of only one kind of atom.

• A. Compound
• B. Atom
• C. Molecule
• D. Element

Answer: (D)

Think about what defines a pure substance by its composition. An element is a pure substance that contains only one kind of atom. That means every particle in the sample is the same type of atom, though those atoms can join to form molecules—for example, oxygen gas (O2) is still an element because both atoms are oxygen. A compound would involve more than one kind of atom chemically bonded together, so it isn’t made of a single kind of atom. A molecule is simply a group of atoms bonded together and can be made from either one element or multiple elements, so it describes structure rather than the purity of the substance. An atom is the basic unit, but the term describing a pure substance consisting of one kind of atom is element.