Atomic Mass Calculator
Our general chemistry calculator computes atomic mass accurately. Enter measurements for results with formulas and error analysis.
Formula
Atomic Mass = Sum of nucleon masses - Mass defect
Atomic mass equals the sum of individual proton, neutron, and electron masses minus the mass defect (mass converted to nuclear binding energy via E=mc2). Mass defect = Total Binding Energy / 931.494 MeV per amu.
Worked Examples
Example 1: Helium-4 Atomic Mass
Problem: Calculate atomic mass of He-4 (2 protons, 2 neutrons, BE/nucleon = 7.074 MeV)
Solution: Sum = 2(1.007276) + 2(1.008665) + 2(0.000549) = 4.032980 amu\nTotal BE = 7.074 * 4 = 28.296 MeV\nMass defect = 28.296/931.494 = 0.030377 amu\nAtomic mass = 4.032980 - 0.030377 = 4.002603 amu
Result: 4.002603 amu (actual: 4.002602 amu)
Example 2: Iron-56 Mass from 10 Grams
Problem: How many moles and atoms in 10g of Fe-56 (26p, 30n)?
Solution: Atomic mass ~ 55.935 amu = 55.935 g/mol\nMoles = 10/55.935 = 0.17878 mol\nAtoms = 0.17878 * 6.022e23 = 1.0765e23 atoms
Result: 0.1788 mol | 1.077e23 atoms
Frequently Asked Questions
What is atomic mass and how is it different from mass number?
Atomic mass is the actual mass of an atom measured in atomic mass units (amu), where 1 amu is defined as 1/12 the mass of a carbon-12 atom. It includes the mass of protons, neutrons, and electrons, minus the mass defect due to nuclear binding energy. Mass number (A) is simply the count of protons plus neutrons and is always a whole number. Atomic mass is always slightly less than the sum of individual nucleon masses because some mass is converted to binding energy according to Einstein's E=mc2. For example, helium-4 has mass number 4 but atomic mass 4.002602 amu.
What is mass defect and binding energy?
Mass defect is the difference between the sum of individual proton and neutron masses and the actual measured mass of the nucleus. This missing mass has been converted to nuclear binding energy that holds the nucleus together, as described by Einstein's mass-energy equivalence E=mc2. The conversion factor is 931.494 MeV per amu. For example, a helium-4 nucleus has a mass defect of 0.03038 amu, corresponding to 28.3 MeV of binding energy. Higher binding energy per nucleon indicates greater nuclear stability, with iron-56 having the maximum at about 8.8 MeV per nucleon.
How is the semi-empirical mass formula used?
The semi-empirical mass formula (Bethe-Weizsacker formula) estimates nuclear binding energy using five terms: volume energy (proportional to A), surface energy (proportional to A^(2/3)), Coulomb energy (proportional to Z^2/A^(1/3)), asymmetry energy (proportional to (N-Z)^2/A), and pairing energy. Each term has an empirically fitted coefficient. This formula successfully predicts binding energies and atomic masses for most nuclides within 1% accuracy, and it explains why nuclei with certain proton-to-neutron ratios are more stable than others.
How do you convert between atomic mass units and grams?
One atomic mass unit equals 1.66054 x 10^-24 grams, which is the reciprocal of Avogadro number expressed in appropriate units. This means the atomic mass in amu is numerically equal to the molar mass in grams per mole. For example, carbon-12 has an atomic mass of exactly 12 amu, and one mole of carbon-12 weighs exactly 12 grams. To find the mass of a single atom in grams, multiply the atomic mass in amu by 1.66054 x 10^-24. To find the number of atoms in a given mass, divide the mass by the molar mass and multiply by Avogadro number.
How do I interpret the result?
Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.
Can I use Atomic Mass Calculator on a mobile device?
Yes. All calculators on NovaCalculator are fully responsive and work on smartphones, tablets, and desktops. The layout adapts automatically to your screen size.