Understanding Mass Defect
Mass defect is the difference between the actual measured mass of an atomic nucleus and the sum of the masses of its individual constituent particles (protons and neutrons) when they are separate. When nucleons combine to form a nucleus, some of their mass is converted into energy, making the resulting nucleus slightly lighter than the sum of its parts.
Relation to Nuclear Binding Energy
This 'lost' mass, known as the mass defect (Δm), is directly equivalent to the nuclear binding energy that holds the nucleus together. According to Einstein's mass-energy equivalence principle (E=mc²), this small amount of mass is converted into a significant amount of energy, which is required to break the nucleus apart into its individual nucleons.
A Practical Example
Consider a helium-4 nucleus, which contains two protons and two neutrons. If you precisely measure the mass of two free protons and two free neutrons, their combined mass will be slightly greater than the measured mass of an intact helium-4 nucleus. This tiny, yet measurable, difference in mass is the mass defect for the helium-4 atom.
Importance in Nuclear Reactions
The concept of mass defect is fundamental to understanding nuclear processes like fission (splitting of heavy nuclei) and fusion (joining of light nuclei). Changes in mass defect during these reactions explain why they release immense amounts of energy, as the mass difference is directly converted into kinetic energy of the products.