This simulation promotes understanding of isotopes by providing a simple way to model isotopes of the first 10 elements in the Periodic Table. In the most basic model, users click on an atomic symbol. The simulation displays a stable isotope for that atom. (For example, choose Helium and view a nucleus with two protons and two neutrons.) Now, drag neutrons into the nucleus and watch to see if atom becomes unstable. Students may be surprised to see that Beryllium and Flourine, for example, are unstable with equal numbers of protons and neutrons in the nucleus. Click on “”Abundance in Nature”” to see how common or rare a particular isotope is in nature. Mass number and Atomic Mass (amu) are displayed in real time. See Related Materials for a lesson plan developed specifically for use with the “”Isotopes and Atomic Mass”” simulation. This item is part of a larger collection of simulations developed by the Physics Education Technology project (PhET).