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Answer to Question #1397 Submitted to "Ask the Experts"Category: Radiation Basics — Binding Energies The following question was answered by an expert in the appropriate field: Q
Why is the mass of an atom of 4He less than the sum of the masses of two protons, two neutrons, and two electrons? Thank you for your time and assistance in this question.
A
When a neutron and a proton are brought close together, they experience a strong, attractive nuclear force. If they bind together to form a nucleus of deuterium, an energy of 2.22 MeV is released. This reaction is observed when a hydrogen nucleus (proton) captures a thermal neutron, releasing a 2.22-MeV gamma photon. This amount of energy is the energy equivalent of the amount of mass observed to be lost when the reaction takes place. The conversion of mass into energy is in accordance with Einstein's relation, E = mc2. In order to separate the neutron and proton again, work in the amount of the binding energy, 2.22 MeV, must be supplied against the attractive nuclear force. This example illustrates the general fact that light nuclei release energy through fusion. Adding a bound electron to the deuterium nucleus to form an atom of deuterium also reduces total mass, but only slightly by comparison, because the much weaker Coulomb force is in play then. By extension, binding of the six constituents in your original question liberates energy and thereby reduces their total mass.
James Turner, CHP, PhD
Answer posted on 16 November 2001. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.
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