A substance of only one type of atom would be a pure substance from any of the elements in the period table. A common example could be nitrogen, N2, which is easily isolated, or gold, Au, which tends not to form compounds with other elements.
When a substance contains only one type of atom, it is called an element. ... All known elements are organized on a chart called the periodic table of elements. An element is a pure substance and is made of only one type of atom; it cannot be broken down into a simpler substance.
