Quickly look up the average atomic mass of chemical elements from the periodic table.
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Enter an element symbol to see its atomic mass
About this calculator
Overview
Atomic mass is the mass of an atom, typically expressed in unified atomic mass units (u). It represents the weighted average of all naturally occurring isotopes of that element.
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Pro Tips
Symbols are case-sensitive (e.g., 'He' for Helium, 'H' for Hydrogen).
Look for the symbol on the Periodic Table if you are unsure.
The mass shown here is the standard atomic weight.
Atomic number (Z) = number of protons.
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Fun Facts
"The atomic mass unit (u) is defined as 1/12th of the mass of a carbon-12 atom."
"Most elements have multiple isotopes, which is why atomic masses are rarely whole numbers - they're weighted averages."
"The periodic table displays atomic mass as the average of all naturally occurring isotopes of that element."
"Nitrogen's atomic mass is 14.007 because nitrogen-14 makes up 99.6% of naturally occurring nitrogen."
"Different isotopes of the same element have nearly identical chemical properties since they share the same number of protons and electrons."
"Oganesson (element 118) has the highest atomic mass of any known element, but it is extremely unstable."
"Technetium (43) and Promethium (61) are the only elements with atomic numbers < 83 that have no stable isotopes."
Formula: Average Atomic Mass = Σ (Isotope Mass × Relative Abundance)
