The Q value is the reaction energy from rest-mass differences: Q = (Σmreactants − Σmproducts)c². When masses are entered in atomic mass units, the conversion is: Q(MeV) = (Δm in u) × 931.49410242.
If you enter mass excess values (MeV), the same idea becomes: Q = ΣΔreactants − ΣΔproducts.
Atomic masses include electrons. If total Z differs, electrons may not cancel. Enabling electron correction adjusts Q by (Zproducts − Zreactants) × 0.51099895 MeV.
- Select an input mode: atomic masses (u) or mass excess (MeV).
- Enter reactant values on the left and product values below.
- Optionally enter Z values to verify charge balance.
- Use advanced rows for extra particles, if needed.
- Press calculate to view Q above the form.
- Download CSV for records or print to PDF.
| Reaction | Σ Reactants (u) | Σ Products (u) | Δm (u) | Q (MeV) | Type |
|---|---|---|---|---|---|
| ²H + ³H → ⁴He + n | 5.030152 | 5.011269 | 0.018883 | 17.59 | Exothermic |
| p + ⁷Li → 2 × ⁴He | 8.024000 | 8.005206 | 0.018794 | 17.50 | Exothermic |
| ¹⁴N + α → ¹⁷O + p | 18.005000 | 18.007000 | -0.002000 | -1.86 | Endothermic |
Understanding Q Value in Nuclear Reactions
The Q value is the net energy change of a nuclear reaction caused by a difference in rest mass between reactants and products. A positive Q means energy is released, while a negative Q means energy must be supplied. Q values are typically reported in MeV and help summarize the reaction’s overall energetics.
Why Mass Differences Create Energy
Mass–energy equivalence links a small mass defect to a measurable energy change. A mass change of 0.001 u corresponds to about 0.9315 MeV. This calculator converts atomic-mass-unit inputs using 931.49410242 MeV/u, so tiny differences in mass can be translated into reaction energy quickly.
Atomic Masses Versus Mass Excess
Atomic masses include bound electrons, while mass excess values are a common tabulated quantity used for nuclei. When total charge is conserved (total Z), electron contributions cancel in atomic-mass calculations. Mass excess mode lets you work directly in MeV without converting from u.
Charge Balance and Electron Correction
Enter Z values to run an automated charge-balance check. If total Z differs, the atomic-mass Q can include a net electron contribution. Enabling correction adjusts Q by (Zproducts − Zreactants) × 0.51099895 MeV to estimate a nucleus-only comparison value.
Interpreting Exothermic and Endothermic Results
Exothermic reactions (Q ≥ 0) can convert rest-mass difference into outgoing kinetic energy or radiation. Endothermic reactions (Q < 0) require incident energy and may be suppressed until sufficient beam energy is available. The sign of Q is a fast indicator of whether a channel is energetically favorable.
Threshold Energy for Endothermic Channels
For a simple two-body endothermic reaction with a stationary target, the laboratory threshold depends on Q and the projectile-to-target mass ratio. When your inputs match that structure, this tool estimates Eth ≈ (−Q) × (1 + mprojectile/mtarget) in MeV for quick screening.
Practical Data Sources and Precision
Use reliable mass tables for precision work, because rounding at the 10−6 u level can shift Q by about 1 keV. Keep units consistent, treat photons as zero mass here, and avoid mixing atomic and nucleus-only conventions unless you understand how your source defines the numbers.
Reporting and Exporting Results
Results are shown in MeV, Joules, and kJ/mol for flexible reporting. Export CSV to preserve inputs and outputs for lab notes, or print to PDF for sharing and archiving. Consistent records make it easier to compare alternate channels and updated mass datasets.
FAQs
1) What does a positive Q value mean?
A positive Q means the products have lower total rest mass than the reactants, so the difference appears as released energy, usually as kinetic energy and sometimes gamma emission.
2) What does a negative Q value mean?
A negative Q means the reaction requires energy input. In experiments, this often implies a threshold beam energy before the reaction channel becomes observable.
3) Should I use atomic masses or mass excess values?
Use atomic masses when your source provides them consistently and Z is conserved. Use mass excess when your dataset is tabulated that way or when you want direct MeV arithmetic.
4) Why does Z balance matter for atomic-mass Q?
Atomic masses include electrons. If total Z is the same on both sides, electron masses cancel. If Z differs, the computed Q can include a net electron contribution unless corrected.
5) When should I apply the electron correction?
Apply it when total Z differs and you want an estimate aligned with nucleus-only conventions. If you are comparing strictly atomic-mass tabulations, leave it unchecked.
6) Why does the threshold value not always appear?
The threshold estimate is shown only for a simple two-reactant, two-product endothermic case with a stationary target assumption. Complex final states need a more detailed kinematic treatment.
7) How accurate are the Joule and kJ/mol outputs?
They use fixed physical constants to convert from MeV, so they are as accurate as your Q value. The dominant uncertainty usually comes from the mass inputs and rounding.