Atomic Mass Definition and Atomic Mass Formula
The atomic mass is the mass of an atom, including its protons, neutrons, and electrons. However, because an electron is 1,836 times less massive than a proton, electrons account for an insignificant fraction of the total mass in most atoms.
While you can state the atomic mass in kilograms, it is more common to express it in atomic mass units (u), also called daltons (Da). One atomic mass unit (one dalton) equals 1/12 the mass of a carbon-12 atom, which makes it equal to approximately 1.66 × 10−27 kg.
To find a reasonable approximation of the atomic mass in atomic mass units, simply add the number of protons and neutrons. So the atomic mass formula is:
A = Z + N
- A — Mass number (total number of protons and neutrons)
- Z — Number of protons in the atom
- N — Number of neutrons in the atom
Note: The actual mass of an atom is slightly less than the sum of its parts due to binding energy mass loss. This mass deficit can be calculated using E = mc². However, this difference is tiny for most atoms, and this calculator does not account for it.
How to Calculate Atomic Mass Using This Calculator
- Enter the number of protons in the atom. The calculator will warn you if you enter a value larger than 118, as this is the highest atomic number observed so far (Oganesson, Og).
- Enter the number of neutrons in the atom. The calculator will warn you if you enter a value larger than 177, as this is the highest neutron count observed in a stable nucleus.
- Choose the unit system: Metric (u, Da, g/mol, kg) or American (amu, g/mol, kg).
- The calculator displays the atomic mass in atomic mass units (u / amu / Da), g/mol, and kilograms, as well as the mass number A.
- Use the quick presets to fill in common isotopes such as Carbon-12, Uranium-235, and Gold-197 with a single click.
Though sounding similar, atomic mass and average atomic mass are different concepts. The average atomic mass is the weighted average of all naturally occurring isotopes of an element. Visit our Average Atomic Mass Calculator to find out more.
Unit Systems: American vs. Metric
| Unit | System | Value (for Carbon-12) |
|---|---|---|
| amu (atomic mass unit) | American | 12 amu |
| u (unified atomic mass unit) | Metric / SI | 12 u |
| Da (dalton) | Metric / SI | 12 Da |
| g/mol | Both | 12 g/mol |
| kg | Metric (SI base) | 1.993 × 10−26 kg |
Numerically, 1 amu = 1 u = 1 Da = 1 g/mol (per atom, relative to the carbon-12 standard). The difference between "amu" and "u" is purely terminological: "amu" is the older American notation, while "u" is the current IUPAC and SI recommendation.
Why Do Atoms Need Neutrons?
An atom has a neutral overall electric charge. However, all of the positive charge is concentrated in the nucleus with the protons. Since like charges repel, there is an electromagnetic force trying to push the nucleus apart. Countering this force is the strong nuclear force — an attractive force that acts between protons and neutrons at very short distances.
By adding neutrons, an atom increases the total amount of attractive strong nuclear force without adding more repulsive electromagnetic force. This is why larger elements need progressively more neutrons than protons to remain stable.
Some atoms are unstable and may only exist for a limited time before undergoing radioactive decay — ejecting particles and transforming into a different element or isotope. You can explore decay rates using our Half-Life Calculator.
FAQs
What is the difference between atomic mass and atomic number?
The atomic number (Z) is the number of protons in an atom and defines which element it is. The atomic mass (A) is the total number of protons and neutrons (also called the mass number) and can vary between isotopes of the same element.
Is atomic mass the same as molecular weight?
No. Atomic mass refers to a single atom. Molecular weight (or molar mass) is the sum of the atomic masses of all atoms in a molecule, expressed in g/mol.
Can atomic mass be a non-integer?
For a single isotope, the atomic mass is approximately an integer (the mass number). It is slightly less due to binding energy. The average atomic mass of a natural element is typically a decimal because it is the weighted average over all naturally occurring isotopes.
What is 1 atomic mass unit in kg?
1 atomic mass unit (1 u = 1 amu = 1 Da) ≈ 1.66053906660 × 10−27 kg.