What term describes a substance comprising only one kind of atom?

• A. Isotope
• B. Mass number
• C. Element
• D. Atomic number

Answer: (C)

A substance made of only one kind of atom is an element. That means all the atoms share the same number of protons, which defines the element by its atomic number. Isotopes are just different forms of the same element that have different numbers of neutrons, so they’re still the same kind of atom overall. Mass number and atomic number describe properties of the atoms themselves (how many protons, how many total nucleons), not the idea of a substance made from a single kind of atom. So the term that best fits a sample containing only one kind of atom is element.