Atomic Mass Calculator
Find atomic mass from protons and neutrons instantly. See units in amu and kilograms, with steps, examples, and helpful notes.
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Helping Notes
- Mass number: \(A = Z + N\). Atomic mass (in u) is well-approximated by \(A\) for most purposes.
- Atomic mass in SI units: \(m_{\text{kg}} = A \times 1\,\text{u}\), with \(1\,\text{u} = 1.66053906660\times10^{-27}\,\text{kg}\).
- Electron mass is negligible for this calculator; we follow the same simplification as the referenced tool.
Results
Mass Number (A)
Atomic Mass (u)
Atomic Mass (kg)
Error
Steps
What Is an Atomic Mass Calculator?
An Atomic Mass Calculator instantly computes the mass of a single atom or isotope and converts units between atomic mass units (u) and kilograms. It links nuclear composition — proton (\(Z\)) and neutron (\(N\)) counts — to precise isotopic masses from reference tables. Atomic mass is a decimal value in u, while the mass number \(A\) is an integer representing total nucleons: \(A = Z + N\). For neutral atoms, a structural model sums constituent particle masses. A refined calculation accounts for the small mass defect caused by nuclear binding energy.
About the Atomic Mass Calculator
The calculator supports two practical modes. Isotopic mass mode computes the exact mass of a nuclide using nuclear composition, tabulated particle masses, and optional binding-energy corrections. Average mode calculates a weighted mean for multiple isotopes with given abundances, producing the familiar periodic-table value. Every step is transparent: identification of \(Z\) and \(N\), substitution of constants, summation, optional mass-defect adjustment, and conversion to kilograms or grams per mole. Common errors are flagged, such as confusing \(A\) with atomic mass or mixing percentage and fractional abundances.
How to Use the Atomic Mass Calculator
- Select Isotopic mass (enter \(Z\), \(N\), or nuclide symbol \(^{A}_{Z}\!X\)) or Average (enter isotopes with abundances).
- Provide constants (\(m_p\), \(m_n\), \(m_e\)); defaults are prefilled from standard references.
- Optionally enter binding energy \(B\) to account for mass defect; otherwise, the structural sum is reported.
- Click calculate to see responsive formulas, step-by-step substitution, and the final mass in u and kg.
- Copy results for lab notebooks, homework, or documentation.
Examples
Example 1: Mass Number and Neutron Count
For \(^{23}_{11}\mathrm{Na}\), \(A = 23\), \(Z = 11 \Rightarrow N = A - Z = 12\).
Example 2: Structural Sum (Neutral Atom)
Approximate \(^{16}\mathrm{O}\) with constants: \(m_p \approx 1.007276\,\text{u}\), \(m_n \approx 1.008665\,\text{u}\), \(m_e \approx 0.00054858\,\text{u}\).
Example 3: Convert to Kilograms
If \(m = 15.999\,\text{u}\), then
Example 4: Optional Weighted Mean
Two isotopes: \(m_1 = 10.0129\,\text{u}\) at 19.9% and \(m_2 = 11.0093\,\text{u}\) at 80.1%:
FAQs
What is the difference between atomic mass and mass number?
Atomic mass is a decimal in u for a specific isotope. Mass number \(A\) is an integer: total protons plus neutrons.
How does atomic mass differ from average atomic mass?
Atomic mass refers to a single isotope. Average atomic mass is a weighted mean across naturally occurring isotopes.
Do I need electron mass?
Yes, for neutral atoms. For ions, adjust the electron count accordingly.
What is mass defect?
The difference between summed particle masses and actual nuclear mass, caused by binding energy \(B/c^2\).
Can I enter a nuclide symbol directly?
Yes. Enter \(^{A}_{Z}\!X\). The calculator extracts \(Z\), \(A\), and computes \(N = A - Z\).
Which constants are used?
Standard values for \(m_p\), \(m_n\), \(m_e\), and \(m_u\). You may override them for sensitivity analysis.
How many significant figures should I report?
Match input precision; four to six significant figures are typical.
Is 1 u the same as g·mol⁻¹?
Numerically yes per atom versus per mole. Units differ (u vs g·mol⁻¹).
Does temperature affect isotopic mass?
No. Isotopic masses are intrinsic; temperature only affects physical state.
Can I compute isotope abundance from a known mean?
Yes. Solve \(\bar m = \sum a_i m_i\) for unknown abundances with \(\sum a_i = 1\).