Online Free Atomic Mass Calculator
Atomic Mass Calculator
Calculate the atomic mass of chemical formulas with precision.
0 / 50
⚛️ Atomic Mass: Definition, Formula, and Examples
🔑 Definition
Atomic mass is the mass of a single atom of a chemical element. It reflects the total mass of protons, neutrons, and electrons, though electrons contribute very little.
- It’s usually expressed in atomic mass units (amu) or Daltons (Da).
- 1 amu is defined as 1/12 the mass of a carbon-12 atom.
👉 Important distinction:
- Atomic mass = mass of one atom (specific isotope).
- Atomic weight (relative atomic mass) = weighted average of all naturally occurring isotopes of an element, based on abundance.
🧮 Formula
For a given isotope:
Atomic Mass≈(Z⋅mp)+(N⋅mn)+(Z⋅me)−Δm\text{Atomic Mass} \approx (Z \cdot m_p) + (N \cdot m_n) + (Z \cdot m_e) – \Delta m
Where:
- ZZ = number of protons
- NN = number of neutrons
- mpm_p = mass of a proton (≈ 1.0073 amu)
- mnm_n = mass of a neutron (≈ 1.0087 amu)
- mem_e = mass of an electron (≈ 0.00055 amu)
- Δm\Delta m = mass defect (binding energy converted from mass)
📊 Examples
| Element | Isotope | Protons | Neutrons | Approx. Atomic Mass (amu) |
|---|---|---|---|---|
| Hydrogen | H-1 | 1 | 0 | 1.0078 |
| Carbon | C-12 | 6 | 6 | 12.0000 (defined standard) |
| Oxygen | O-16 | 8 | 8 | 15.9949 |
| Uranium | U-238 | 92 | 146 | 238.0508 |
🌍 Atomic Mass vs Atomic Weight
- Atomic mass: exact mass of a specific isotope (e.g., C-12 = 12.0000 amu).
- Atomic weight: average mass of all isotopes in nature, weighted by abundance.
- Example: Carbon’s atomic weight = 12.011 amu (because ~99% is C-12, ~1% is C-13).
In the U.S. education system, the Periodic Table typically lists atomic weight (average), not the mass of a single isotope.
⚖️ Why Atomic Mass Matters
- Chemistry: Determines molar mass and stoichiometry in reactions.
- Physics: Connects to nuclear binding energy and stability.
- Medicine: Isotopes with different atomic masses are used in imaging and treatment.
- Agriculture & Industry: Stable isotopes help trace nutrient cycles and optimize fertilizers.
FAQ
Q: Is atomic mass always a whole number? No. Because of the mass defect and isotope variation, atomic masses are usually decimals.
Q: Why is carbon-12 special? It’s the reference standard: 1 amu = 1/12 the mass of a carbon-12 atom.
Q: How is atomic mass measured? Using mass spectrometry, which separates isotopes by mass-to-charge ratio.