Periodic Table with Atomic Mass
Standard atomic weights to three decimal places, in u (equal to g/mol). Bracketed values are the mass number of the most stable isotope.
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Using atomic mass in calculations
Atomic mass is a bridge between atoms and grams. One mole of any element (6.022 × 10²³ atoms) has a mass in grams equal to its atomic mass. For water, H₂O: 2 × 1.008 + 15.999 = 18.015 g/mol, so 18 g of water holds one mole of molecules.
Most classes round to two decimals or to the nearest tenth. Round only at the end of a calculation, or the error adds up.
Atomic masses of all elements
Questions students ask
What is atomic mass?
The average mass of an element’s atoms, weighted by how common each isotope is in nature, measured in unified atomic mass units (u, also called daltons). Chlorine is 35.45 u because about three-quarters of its atoms are chlorine-35 and one quarter chlorine-37.
Is atomic mass the same as molar mass?
Numerically, yes. An element’s atomic mass in u equals the mass of one mole of its atoms in grams. Carbon is 12.011 u per atom and 12.011 g/mol. Add them up for a compound with the molar mass calculator.
Why are some masses in brackets?
Elements with no stable isotopes have no natural isotope mix to average. Tables give the mass number of the longest-lived isotope in square brackets instead, e.g. [244] for plutonium.
What is the difference between atomic mass and mass number?
Mass number is a whole-number count of protons plus neutrons in one specific isotope. Atomic mass is the weighted average over all natural isotopes and is rarely a whole number.