Answer: An atom is the basic unit of a chemical element.
An isotope is each of two or more forms of the same element that contain equal numbers of protons
Mass number is the total number of protons and neutrons in a nucleus.
Step-by-step explanation:
An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. An isotope is each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element.
Mass number is the total number of protons and neutrons in a nucleus.