What distinguishes an element from a compound, and how is a chemical formula for a compound written?

• A. An element contains two or more types of atoms; a compound contains only one type of atom; chemical formulas show ratios.
• B. An element contains only one type of atom; a compound contains two or more elements in fixed proportions; chemical formulas show the types and numbers of atoms (e.g., H2O).
• C. An element is a pure substance; a compound is a mixture.
• D. An element has a chemical formula; a compound has density.

Answer: (B)

Elements are pure substances made of only one type of atom, while compounds are pure substances formed when two or more elements bond together in fixed proportions. A chemical formula for a compound shows exactly which elements are present and how many atoms of each are in one unit of the compound. For example, in water the formula H2O means there are two hydrogen atoms and one oxygen atom in each molecule. The subscripts indicate the numbers of atoms, and if there’s no subscript after an element, it means there’s one atom of that element. This fixed ratio comes from the way atoms bond to form stable substances, which is why a compound like H2O has a specific composition rather than any arbitrary mix of elements.