Final answer:
If a substance analyzed using an electron microscope contains only one type of atom, it is classified as an element, which is a pure substance made up of only one kind of atom.
Step-by-step explanation:
When a substance is analyzed using an electron microscope and is found to contain only one type of atom, this substance is an element. An element is a pure substance that cannot be broken down into other types of substances. Each element is made up of just one type of atom.
Since we have a substance with only one type of atom, it does not match the definition of a compound, which consists of two or more different types of atoms. It also cannot be a mixture because mixtures comprise two or more substances mixed together but not chemically bonded, and isotopes are variations of the same element with different numbers of neutrons. Hence, we can conclude that the substance in question is indeed an element.