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Mass Defect
Mass defect is the difference between the total mass of the separate nucleons and the actual mass of the bound nucleus.
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Mass Defect
A bound nucleus weighs slightly less than all its protons and neutrons would weigh separately. That difference is called mass defect.
Recommended for Class 12
Mass Defect
Definition
Mass defect is the difference between the total mass of the separate nucleons and the actual mass of the bound nucleus.
Δm = mass of separate nucleons − mass of nucleus
Notes — Intermediate
If the masses of all individual protons and neutrons are added, their total is slightly greater than the measured mass of the nucleus they form.
This difference is the mass defect.
For a nucleus containing `Z` protons and `N` neutrons:
**Δm = Zmₚ + Nmₙ − mnucleus**
The missing mass has not simply disappeared. It corresponds to the energy released when the nucleus was formed.
Using Einstein's relation:
**E = Δmc²**
the mass defect can be converted into nuclear binding energy.
Mass Defect — Advanced
Mass defect reflects the lower total rest energy of a bound nuclear system relative to its separated constituent nucleons.
Common Mistakes
Common Mistakes
- Mistake
Do not interpret mass defect as missing matter. It represents a difference in total rest energy. Keep mass units consistent before calculation.
Revision Zone
60-Second Revision
Quick cues from this topic — use them for last-minute revision.
Separate nucleons − nucleus = `Δm`
`Δm` → binding energy.
The actual mass of a bound nucleus is less than the sum of the masses of its separate protons and neutrons. This difference is called mass defect. It represents the energy released during formation of the nucleus. Through `E=Δmc²`, mass defect gives the nuclear binding energy.
- Definition
- The actual mass of a bound nucleus is less than the sum of the masses of its separate protons and neutrons. This difference is called mass defect. It represents the energy released during formation of the nucleus. Through `E=Δmc²`, mass defect gives the nuclear binding energy.
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Practice
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Q1. Define mass defect
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Q2. Write its formula
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Q3. Why does mass defect occur?
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Q4. Which equation converts it to energy?
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Q5. Does mass simply disappear?
Exam Practice
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Q1. Define and explain mass defect
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Q2. Write an expression for nuclear mass defect
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Q3. Explain its connection with binding energy
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Q4. Solve a basic mass-defect problem
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Q5. Explain mass-energy accounting in nuclear binding
School-standard facts for Matric and Intermediate. Original explanations — not copied from textbooks.