Mass Deficiency
Definition and meaning of Mass Deficiency in chemistry.
Mass deficiency is the small difference between the sum of the individual masses of separated protons and neutrons and the actual measured mass of the nucleus they form. This mass loss arises because energy is released when nucleons bind together, and this binding energy corresponds to mass loss via Einstein's E = mc².
In more detail
When protons and neutrons combine to form a nucleus, the strong nuclear force attracts them and releases binding energy. By Einstein's mass-energy equivalence, this released energy appears as a decrease in the total mass of the system. The greater the binding energy per nucleon, the larger the mass deficiency and the more stable the nucleus. Mid-mass nuclei in the iron-nickel region show the greatest mass deficiency per nucleon and are among the most stable found in nature; nickel-62 has the single highest binding energy per nucleon of any nuclide, though iron-56 is the classic textbook example because it is by far the most abundant nuclide in that stability peak, formed as the endpoint of stellar fusion.
Key facts
| Field | Physical Chemistry |
|---|---|
| Formula (example nucleus) | He-4 |
| Relationship to binding energy | Δm = BE/c² (binding energy converted to mass) |
| Typical values | ~0.002 amu for light nuclei (e.g., deuterium); up to a few amu for heavy nuclei (e.g., ~1.9 amu for uranium-238) |
Helium-4 (He-4) demonstrates mass deficiency clearly. Four separated nucleons (2 protons + 2 neutrons) have a combined mass of approximately 4.0319 amu, but the actual He-4 nucleus (the bare alpha particle, without its 2 orbital electrons) has a mass of only about 4.0015 amu, yielding a mass deficiency of about 0.0304 amu (equivalent to about 28.3 MeV of binding energy, or roughly 7.07 MeV per nucleon).
Frequently asked questions
Why does mass deficiency occur?
The strong nuclear force binds nucleons together and releases binding energy. By E = mc², this energy loss manifests as a decrease in the total mass of the nucleus.
How is mass deficiency calculated?
Subtract the actual nucleus mass (from mass spectrometry) from the sum of the separated nucleon masses. The result is typically expressed in atomic mass units (amu) or converted to binding energy in megaelectronvolts (MeV).