Mass Defect Definition in Physics and Chemistry

Mass is associated with the binding energy between nucleons

A mass defect occurs when the mass of an atom is different from the sum of the masses of its subatomic particles.
A mass defect occurs when the mass of an atom is different from the sum of the masses of its subatomic particles. RICHARD KAIL / Getty Images
By
Anne Marie Helmenstine, Ph.D.
Anne Marie Helmenstine, Ph.D.
Chemistry Expert
  • Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
  • B.A., Physics and Mathematics, Hastings College
Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
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Updated on November 06, 2019

In physics and chemistry, a mass defect refers to the difference in mass between an atom and the sum of the masses of the protons, neutrons, and electrons of the atom.
This mass is typically associated with the binding energy between nucleons. The "missing" mass is the energy released by the formation of the atomic nucleus. Einstein's formula, E = mc2, may be applied to calculate the binding energy of a nucleus. According to the formula, when energy increases, mass and inertia increase. Removing energy reduces mass.

Key Takeaways: Mass Defect Definition

  • A mass defect is the difference between an atom's mass and the sum of the masses of its protons, neutrons, and electrons.
  • The reason the actual mass is different from the masses of the components is because some of the mass is released as energy when protons and neutrons bind in the atomic nucleus. Thus, the mass defect results in a lower-than-expected mass.
  • The mass defect follows the conservation laws, where the sum of mass and energy of a system is constant, but matter can be converted into energy.

Mass Defect Example

For example, a helium atom containing two protons and two neutrons (four nucleons) has a mass about 0.8 percent lower than the total mass of four hydrogen nuclei, which each contain one nucleon.

Sources

  • Lilley, J.S. (2006). Nuclear Physics: Principles and Applications (Repr. with corrections Jan. 2006. ed.). Chichester: J. Wiley. ISBN 0-471-97936-8.
  • Pourshahian, Soheil (2017). "Mass Defect from Nuclear Physics to Mass Spectral Analysis." Journal of The American Society for Mass Spectrometry. 28 (9): 1836–1843. doi:10.1007/s13361-017-1741-9
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Helmenstine, Anne Marie, Ph.D. "Mass Defect Definition in Physics and Chemistry." ThoughtCo, Aug. 27, 2020, thoughtco.com/definition-of-mass-defect-605328. Helmenstine, Anne Marie, Ph.D. (2020, August 27). Mass Defect Definition in Physics and Chemistry. Retrieved from https://www.thoughtco.com/definition-of-mass-defect-605328 Helmenstine, Anne Marie, Ph.D. "Mass Defect Definition in Physics and Chemistry." ThoughtCo. https://www.thoughtco.com/definition-of-mass-defect-605328 (accessed June 1, 2023).