Where does the missing mass go in a nuclear reaction?

In a nuclear reaction, the "missing mass," often referred to as the mass defect, is not lost but is converted directly into a tremendous amount of energy.

This phenomenon is a fundamental principle of modern physics, most famously described by Albert Einstein's equation, E=mc². This equation reveals the intrinsic relationship between mass and energy, demonstrating that mass can be converted into energy and vice versa.

• E represents energy (Joules).
• m represents mass (kilograms).
• c represents the speed of light in a vacuum (approximately 3 x 10⁸ meters per second).

Because the speed of light (c) is an extraordinarily large number, and it is squared in the equation (c²), even a tiny amount of "missing mass" (m) results in an immense release of energy (E).

The Principle of Mass-Energy Conversion

During a nuclear reaction, such as fusion or fission, the total mass of the resulting particles (nuclei, neutrons, etc.) is slightly less than the total mass of the initial particles. This reduction in mass—the "missing mass" or mass defect—is precisely the amount of mass that has been transformed into energy. This released energy manifests as the kinetic energy of the reaction products, heat, and sometimes as gamma rays.

Nuclear Fission Explained

Nuclear fission is a process where a heavy atomic nucleus splits into two or more smaller, lighter nuclei.

• Process: Typically, a heavy, unstable nucleus (like Uranium-235 or Plutonium-239) is struck by a neutron, causing it to split into smaller nuclei, additional neutrons, and a significant amount of energy.
• Mass-Energy Conversion: The sum of the masses of the fission products (the lighter nuclei and emitted neutrons) is less than the mass of the original heavy nucleus. This mass difference is converted into energy.
• Practical Application: This controlled chain reaction is the basis for nuclear power plants, which generate electricity by harnessing the heat produced from fission. Learn more about nuclear fission from the Department of Energy.

Nuclear Fusion Explained

Nuclear fusion is the process in which two light atomic nuclei combine to form a single, heavier nucleus.

• Process: For instance, two isotopes of hydrogen, deuterium and tritium, can fuse under extreme temperature and pressure to form a helium nucleus and a neutron.
• Mass-Energy Conversion: The mass of the newly formed heavier nucleus is less than the combined mass of the original lighter nuclei. This "leftover mass" is released as a massive burst of energy.
• Cosmic Significance: Nuclear fusion is the power source of the sun and other stars, where gravitational forces create the necessary conditions for light elements to fuse and release energy. Researchers are working to replicate this process on Earth as a potential source of clean, abundant energy. Explore more about nuclear fusion from ITER, the international fusion energy project.

Simplified Mass Comparison in a Nuclear Reaction

To illustrate the mass defect:

This "missing mass" of 0.010 units is the mass that has been transformed into the energy released by the nuclear reaction.

Why Does This Happen? The Role of Binding Energy

The fundamental reason for the mass defect and the accompanying energy release lies in the binding energy of atomic nuclei. Nucleons (protons and neutrons) within a nucleus are held together by the strong nuclear force. When lighter nuclei fuse or heavier nuclei undergo fission, the rearrangement of these nucleons can lead to a more stable configuration. A more stable nucleus has a higher binding energy per nucleon, meaning its constituent particles are more tightly bound together. This increased binding energy corresponds to a lower total mass for the nucleus—the mass difference being converted into the energy released.

The immense energy yields from nuclear reactions underscore the power of mass-energy conversion, making them central to both natural phenomena like stellar processes and human technologies such as nuclear power generation.