Formula used
Initial excitation energy increases available energy. Final excitation energy reduces the ground-state Q value.
How to use this calculator
- Choose automatic, mass-defect, binding-energy, or mass-excess mode.
- Add every reactant and product with its count, A value, Z value, data type, value, and unit.
- Use consistent atomic or nuclear masses unless you understand the needed electron corrections.
- Enable threshold or two-product sharing options when their assumptions match your reaction.
- Review balance warnings, then calculate and export the result.
Worked example
For deuterium plus tritium producing helium-4 and a neutron, the entered atomic masses give a positive mass defect. Multiplying that defect by the atomic-mass energy constant produces about 17.6 MeV. The positive sign identifies an exothermic fusion reaction.
| Reaction | Typical Q value | Classification | Notes |
|---|---|---|---|
| ²H + ³H → ⁴He + n | About 17.6 MeV | Exothermic | Two-body energy sharing is available. |
| n + ¹H → ²H + γ | About 2.22 MeV | Exothermic | Gamma energy carries most released energy. |
| ²³⁸U → ²³⁴Th + ⁴He | About 4.27 MeV | Exothermic | Alpha decay example. |
| ¹⁴C → ¹⁴N + e⁻ + anti-neutrino | About 0.156 MeV | Exothermic | Atomic-mass handling needs care. |
Important interpretation notes
A positive Q value means rest-mass energy becomes kinetic energy, radiation, or excitation. A negative Q value means the reaction needs external energy. A zero value means no meaningful net rest-mass change.
The Q value is not automatically one particle’s kinetic energy. Momentum conservation determines how released energy is shared. Multi-particle final states require more complete kinematic analysis.
Atomic masses include bound electrons, while nuclear masses do not. Electron counts often cancel in balanced reactions, but beta decay and electron capture need special treatment. Evaluated nuclear-data tables should be used for precision work.
Frequently asked questions
What does a positive Q value mean?
It means the reaction releases energy and is exothermic.
What does a negative Q value mean?
It means the reaction is endothermic and requires input energy.
Can atomic and nuclear masses be mixed?
They should not be mixed unless electron masses are handled explicitly.
Why can threshold energy exceed the magnitude of Q?
Momentum conservation requires some incident energy to remain as center-of-mass motion.
Does the calculator include excited states?
Yes. Initial excitation raises Q, while final excitation lowers it.
How are beta decays handled?
Enter all relevant particles and use consistent mass conventions.
Can gamma photons be entered?
Yes. Use zero rest mass and represent known gamma energy separately when appropriate.
Is neutrino mass included?
Basic calculations commonly neglect its tiny rest-mass contribution.
Is this suitable for professional nuclear-data evaluation?
No. It is an educational estimator and should be checked against evaluated data.