1. Mass Defect and Einstein's Equation
Everyone knows of Einstein's famous equation but very few actually use it. It is really important when we look at nuclear physics. In chemical reactions we assume that mass is conserved but that is not case when we look at the particles that make up the nucleus. Their total mass when they join together is less than their individual masses: this 'mass defect' is released as energy when they combine.
2. Binding Energy and why things Fuse or Fission
When individual protons and neutrons bind together their total mass is less than their original masses. We can investigate the average binding energy per nucleon and this then tells us about the more stable elements and why some nuclei can undergo fusion or fission, releasing more energy in the process as they aim to be as stable as iron.