What describes a substance formed when two or more elements combine?

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

Answer: (D)

When different elements join together through chemical bonding to form a substance with a definite composition, that substance is a compound. The defining feature is a fixed ratio of elements, giving the compound properties distinct from its constituent elements. For example, water arises from hydrogen and oxygen in a specific 2:1 ratio, resulting in a substance with its own unique characteristics. A molecule is the smallest unit that can exist on its own and can be either a single element (like O2) or a compound (like H2O), so not every molecule is a compound even though compounds are often molecules. An atom is the basic unit of a single element, and an ion is a charged particle created by electron transfer. The scenario described—two or more elements combining to form a new substance—points to a compound.