The document discusses nuclear chemistry concepts including mass defect, nuclear stability, and radioactivity. It explains that the mass of an atom is less than the sum of its parts due to the conversion of mass to nuclear binding energy during nucleus formation. This mass difference is called the mass defect. Examples of nuclear reactions like alpha, beta, and gamma emission are provided. Applications of nuclear chemistry such as nuclear power generation, radiocarbon dating, food irradiation, and nuclear medicine are also summarized.