What are Isotopes?
Atoms are composed of a cloud of electrons surrounding a dense nucleus that is 100,000 times smaller. The nucleus is comprised of protons and neutrons, each with mass roughly 2000 times that of the electron. The number of protons — the "atomic number" — determines the element; for example, a Strontium nucleus always has 38 protons, and a Rubidium nucleus always has 37. There is an equal number of electrons surrounding the nucleus to keep the atom electrically neutral, and these electrons determine the chemical properties of the element — such as, forming the bonds that enable molecules like Strontium Chloride SrCl2 to form from individual Strontium and Chlorine atoms.
The different isotopes of an element have different number of neutrons; for example, while most (82.58%) of Strontium atoms have 50 neutrons, some have only 46, 48, or 49. For only certain numbers of neutrons, atoms are "stable" and do not radioactively decay into another element. For Strontium, the stable isotopes have atomic mass 84 [38 protons and 46 neutrons], 86 [38 protons and 48 neutrons], 87, and 88. For other numbers of neutrons, the atoms are unstable and radioactively decay. For example, Strontium-82 is radioactive and decays to Rubidium-82 with a half-life of 25 days. For this reason, Strontium is used in generators to provide Rubidium-82 — the most convenient Positron Emission Tomography (PET) agent in heart imaging — by radioactive decay.