The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope. A number of artificial radioactive isotopes of cesium are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation.esium is a member of the alkali family, which consists of elements in Group 1 (IA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. The alkalis include lithium, sodium, potassium, rubidium, and francium.