What is the difference between atomic mass and mass number?

Mass number (A) is the integer count of protons + neutrons. Atomic mass is the precise mass in amu, which accounts for binding energy and is not exactly an integer.

Why are atomic masses not whole numbers?

Two reasons: (1) standard atomic mass is a weighted average of isotopes, and (2) even individual isotope masses aren't exactly integers due to mass defect (nuclear binding energy). This keeps planning practical and lowers the chance of preventable errors.

What is 1 amu in grams?

1 amu = 1.66054 × 10⁻²⁴ g. This equals 1/(6.022 × 10²³), the reciprocal of Avogadro's number divided by the gram.

Mass defect is the difference between the sum of individual particle masses and the actual nuclear mass. This "missing mass" has been converted to binding energy (E = mc²) that holds the nucleus together.

Which element is the atomic mass standard?

Carbon-12 (¹²C) is the standard: 1 amu = exactly 1/12 of the mass of a ¹²C atom. This keeps planning practical and lowers the chance of preventable errors.

Why does chlorine have atomic mass 35.45?

Chlorine has two stable isotopes: ³⁵Cl (75.77%) and ³⁷Cl (24.23%). The weighted average is 0.7577 × 34.969 + 0.2423 × 36.966 = 35.45 amu.

Atomic Mass Calculator

Look up atomic masses for all 118 elements. Calculate formula masses, convert between amu and grams, and explore mass defect and nuclear binding energy.

Select Element

Element

Input Mode

Number of Atoms

atoms

Mass Number OverrideFor specific isotope (or leave blank for standard)

Atomic Mass

55.8450 amu

= 55.8450 g/mol

Mass in Grams

55.8450 g

= 0.0558 kg

Moles

1.000000

1 mol = 6.022e+23 atoms

Number of Atoms

6.0220e+23

Total atoms in the sample

Mass Defect

0.60408 amu

Expected mass - actual mass

Binding Energy

562.70 MeV

10.048 MeV/nucleon

Binding Energy per Nucleon

6.82 MeV

6.59 MeV

6.98 MeV

7.46 MeV

7.87 MeV

-4.41 MeV

8.07 MeV

4.56 MeV

8.23 MeV

5.68 MeV

-4.61 MeV

5.10 MeV

5.61 MeV

10.05 MeV

-0.60 MeV

2.03 MeV

8.65 MeV

7.71 MeV

11.14 MeV

7.46 MeV

Atomic Mass Table

Z	Symbol	Name	Atomic Mass (amu)	Mass # (A)
1	H	Hydrogen	1.0079	1
2	He	Helium	4.0026	4
6	C	Carbon	12.0107	12
7	N	Nitrogen	14.0067	14
8	O	Oxygen	15.9994	16
11	Na	Sodium	22.9898	23
12	Mg	Magnesium	24.3050	24
13	Al	Aluminium	26.9820	27
14	Si	Silicon	28.0860	28
15	P	Phosphorus	30.9740	31
16	S	Sulfur	32.0600	32
17	Cl	Chlorine	35.4530	35
19	K	Potassium	39.0980	39
20	Ca	Calcium	40.0780	40
26	Fe	Iron	55.8450	56
29	Cu	Copper	63.5460	63
30	Zn	Zinc	65.3800	65
47	Ag	Silver	107.8680	108
79	Au	Gold	196.9670	197
82	Pb	Lead	207.2000	208
92	U	Uranium	238.0290	238

Planning notes, formulas, and examples

About the Atomic Mass Calculator

Atomic mass is the mass of an atom measured in atomic mass units (amu or u), where 1 amu is defined as exactly 1/12 the mass of a carbon-12 atom. The standard atomic weight listed on the periodic table is a weighted average of the masses of all naturally occurring isotopes of an element, which is why it is rarely an integer.

Atomic mass is fundamental to quantitative chemistry. It connects the atomic scale to the macroscopic world through Avogadro's number: the atomic mass in amu numerically equals the molar mass in g/mol. This means 12.011 amu of carbon weighs 12.011 g per mole, which is 6.022 × 10²³ atoms.

It gives precise atomic masses for all elements, converts between amu and grams/kilograms, calculates the mass of a specified number of atoms, and explores the mass defect — the difference between the sum of individual proton and neutron masses and the actual nuclear mass, which accounts for nuclear binding energy via E = mc².

When This Page Helps

Quickly look up precise atomic masses and perform conversions between amu, grams, moles, and number of atoms. The mass defect calculator connects atomic mass to nuclear physics.

How to Use the Inputs

Select an element from the dropdown or search by symbol/name.
View the standard atomic mass and common isotope masses.
Enter a number of atoms or moles for mass conversion.
Toggle between amu, grams, and kilograms.
Explore mass defect by entering the mass number of a specific isotope.
Use preset elements for quick access.
Compare atomic masses across the periodic table.

Formula used

Standard Atomic Mass = Σ (fractional abundance × isotopic mass)

Mass of N atoms = N × atomic mass (amu)
Mass in grams = moles × molar mass
1 amu = 1.66054 × 10⁻²⁴ g

Mass defect: Δm = Z×m_p + N×m_n - m_atom
Binding energy: E = Δm × c² = Δm × 931.5 MeV/amu

Example Calculation

Result: 10.55 g

Copper has atomic mass 63.546 amu. For 10²³ atoms: mass = 10²³ × 63.546 amu × 1.66054×10⁻²⁴ g/amu = 10.55 g. Equivalently, 10²³ atoms = 0.1661 mol, and 0.1661 × 63.546 g/mol = 10.55 g.

Tips & Best Practices

The atomic mass in amu numerically equals the molar mass in g/mol — this is by definition.
Relative atomic mass is dimensionless; atomic mass has units of amu.
Iron-56 has the highest binding energy per nucleon, making it the most stable nucleus.
Mass spectrometry directly measures atomic and molecular masses with very high precision.
For radioactive elements without stable isotopes, the mass of the longest-lived isotope is often listed in brackets.
Hydrogen-1 has a mass of 1.00783 amu, slightly more than 1 because even it has a binding energy (proton mass ≠ 1 amu exactly).

The Atomic Mass Scale

The modern atomic mass scale is based on carbon-12. Before 1961, two different scales existed: chemists used oxygen-16 and physicists used oxygen as a mixture of isotopes. Unification on carbon-12 was a compromise that pleased both communities (the maximum shift in any element's mass was only 0.004%).

Mass Defect and Nuclear Energy

Einstein's famous equation E = mc² has a direct chemical application in the mass defect. When nucleons bind together, they release energy and the resulting nucleus weighs slightly less than the sum of its parts. For ⁵⁶Fe, the mass defect is ~0.53 amu or ~492 MeV — enough to accelerate a proton to 99.99% the speed of light.

Isotope Applications and Measurement

Mass spectrometers separate isotopes by their mass-to-charge ratio and can measure atomic masses to seven decimal places. Applications include geological dating (U-Pb, K-Ar methods), forensic analysis (stable isotope ratios), medical diagnostics (PET scans using ¹⁸F), and nuclear energy (²³⁵U enrichment).

Sources & Methodology

Last updated: June 1, 2026

Frequently Asked Questions

Mass number (A) is the integer count of protons + neutrons. Atomic mass is the precise mass in amu, which accounts for binding energy and is not exactly an integer.