AI Tutor
If you weigh two loose protons and two loose neutrons, then pack them together into a helium nucleus and weigh it again, the intact nucleus is measurably lighter than its parts.
No particles leaked out or broke apart during the process—yet some of the original mass has vanished.
Where did that missing mass go?

The Missing Mass: Mass Defect

The difference between the total mass of individual isolated nucleons (protons and neutrons) and the actual mass of the bound nucleus is called the mass defect (ΔmΔm).
Nuclear chemists measure these tiny quantities in atomic mass units (u\text{u}), where an isolated proton has a mass of 1.00728 u1.00728\text{ u} and an isolated neutron has a mass of 1.00866 u1.00866\text{ u}.
How can physical matter disappear simply by binding particles together?

Einstein's Mass-Energy Equivalence

In 1905, Albert Einstein solved this puzzle by demonstrating that mass and energy are two interchangeable forms of the same fundamental physical quantity.
When separate nucleons snap together, strong nuclear attraction pulls them tightly, releasing stored potential energy as nuclear binding energy—and that released energy comes directly from the converted mass.
Einstein formulated this relationship with the equation E=mc2E = mc^2, where EE is energy in Joules (J\text{J}), mm is the mass in kilograms (kg\text{kg}), and cc is the speed of light in a vacuum (3.00×108 m/s3.00 \times 10^8\text{ m/s}).