Radioactive isotopes are used for blood flow monitoring, cancer treatment, paper mills, carbon dating and smoke alarms. Each isotope used in these applications has a characteristic half-life.
Isotopes of the same element are alike except for the number of neutrons. As a result, isotopes of the same element have different masses. In the same way, pennies of different years are still all pennies, but differ in masses due to their mint dates and material make-up, just like isotopes 3.