Which term describes a pure substance that is made up of only one type of atom?

A pure substance is something that occurs in nature. An element is made up of one type of atom only and cannot be split further. A compound has the combined properties of the elements from which it is made. Atoms show the macro properties of an element.

when a reversible reaction is occurring in both directions at the same rate; the products and reactants do not change in concentration. a chemical reaction where the compound is broken into its constituent parts.