Nuclear binding energy is the energy required to split an atom's nucleus into smaller nuclei or nucleons. It is equal to the mass defect (the difference between the total mass of nucleons and the actual mass of the nucleus) times the speed of light squared. Binding energy varies with mass number, being highest for mid-sized nuclei and lower for light and heavy nuclei. Elements with higher binding energy per nucleon are more stable as their nuclei are more tightly bound.