Which term describes a substance in which all atoms are the same kind?

• A. Compound
• B. Element
• C. Pure substance
• D. Mixture

Answer: (B)

A substance that has only one kind of atom is called an element. Elements are the simplest forms of matter with a unique type of atom, and they can’t be broken down into simpler substances by chemical means. That’s why a pure substance can be either an element or a compound, but the defining feature of an element is that every particle is the same kind of atom. For example, oxygen is made of oxygen atoms, and gold is made of gold atoms. In contrast, a compound contains two or more different elements chemically bonded together (like water, which has hydrogen and oxygen in a fixed ratio). A mixture is a combination of substances that are not chemically bonded and can have varying composition (like air or a salad). So the term for a substance where all atoms are the same kind is element.