How this instrument works
An element's standard atomic weight is the average mass of its atoms, weighted by how abundant each of its naturally occurring isotopes actually is, expressed in grams per mole (equivalently, unified atomic mass units per atom). It's not the mass of any single atom — most elements exist as a mix of isotopes with slightly different masses, and the standard atomic weight is the abundance-weighted average across that natural mix, which is why chlorine's atomic weight (35.45) falls between its two main isotopes' masses (35 and 37) rather than matching either exactly.
This lookup uses the IUPAC Commission on Isotopic Abundances and Atomic Weights' (CIAAW) 2021 standard atomic weights, published in Pure and Applied Chemistry in 2022 — the internationally recognized reference values used throughout chemistry for converting between mass and moles. Values here are the conventional, abridged single-number weights (typically five significant figures) rather than the technically more precise isotopic-abundance intervals CIAAW also publishes for elements whose natural isotope ratio varies by source material.
The table covers a curated set of 50 commonly used elements, spanning hydrogen (1.008 g/mol) up through uranium (238.03 g/mol) by atomic number — not a complete, contiguous run of every element in that span, since less commonly referenced entries such as the lanthanides and several second- and third-row transition metals are left out. Unlike this site's electronegativity lookup, which excludes helium, neon, and argon for a specific chemical reason (no bonding behavior to measure), the omissions here are purely a matter of which elements come up often enough in general chemistry to include, not missing or unmeasurable data — every element does have a well-defined standard atomic weight, whether or not it appears in this particular table.
- Open the 'Element' dropdown and select the element you need, listed as symbol and full name, e.g. 'C — Carbon.'
- Read 'Standard atomic weight (g/mol)' below — it updates instantly as you change the element.
- Use this value directly to convert between grams and moles for that element: moles = mass in grams ÷ atomic weight.
- For a compound rather than a single element, look up each element's atomic weight separately and sum them according to the compound's formula.
Worked example — carbon and uranium, opposite ends of the table
Select 'C — Carbon' from the dropdown. The result reads 12.011 g/mol — one of the most-cited numbers in chemistry, and the historical basis of the atomic mass unit itself, since the modern unified atomic mass unit is defined so that carbon-12 (the most abundant carbon isotope) weighs exactly 12 u.
Now switch to 'U — Uranium,' the heaviest element in this lookup. The result reads 238.0289 g/mol — about 20 times heavier than carbon per mole, reflecting uranium's much larger, more tightly packed nucleus (92 protons plus roughly 143-146 neutrons across its natural isotopes, versus carbon's 6 protons and typically 6 neutrons).
Questions
What's the difference between atomic mass and atomic weight?
Strictly, 'atomic mass' refers to the mass of one specific isotope of an element (like carbon-12's mass of exactly 12 u), while 'atomic weight' — more precisely, 'standard atomic weight' — is the abundance-weighted average across all of that element's naturally occurring isotopes. In everyday chemistry the two terms get used interchangeably, including in this calculator's name, but the periodic table number you look up for stoichiometry (like carbon's 12.011) is technically the standard atomic weight, not the mass of any single atom.
Why isn't carbon's atomic weight exactly 12?
Because natural carbon isn't pure carbon-12 — it's a mix of isotopes, overwhelmingly carbon-12 (about 98.9%) but with a small fraction of carbon-13 (about 1.1%), which is heavier. The standard atomic weight of 12.011 g/mol is the abundance-weighted average of that natural mix. Carbon-12 itself is defined as exactly 12 u by convention — it's the anchor the entire atomic mass scale is built from — but natural carbon as you'd actually weigh it in a sample is very slightly heavier than 12 because of that carbon-13 contribution.
Does this calculator cover every element on the periodic table?
No — it covers a curated set of 50 commonly used elements, spanning hydrogen through uranium by atomic number but skipping roughly 40 elements within that span, including the full lanthanide series, most second- and third-row transition metals (yttrium through palladium, for instance), and a handful of others like indium and thallium. That's deliberately narrower than the full periodic table (118 elements), but broad enough to cover essentially every element encountered in general and introductory chemistry coursework, common compounds, and everyday lab work; beyond hydrogen–uranium, it also excludes every actinide past uranium and every synthetic superheavy element.
Why does the source distinguish 'abridged' atomic weights from full IUPAC intervals?
For some elements — hydrogen, lithium, boron, carbon, and others — natural isotopic abundance actually varies measurably depending on where a sample came from, so IUPAC technically publishes a range (an interval) rather than one fixed number for full scientific rigor. For everyday use, IUPAC also publishes a single 'abridged' conventional value for each element, generally the interval's midpoint rounded to about five significant figures, which is what this lookup provides — accurate enough for essentially all standard stoichiometry and lab calculations.
How do I find the molar mass of a compound, not just a single element?
Look up each element in the compound separately here, multiply each one's atomic weight by how many atoms of it appear in the formula, and add the results together. For water (H2O), for example: 2 × hydrogen (2 × 1.008) + 1 × oxygen (15.999) = 2.016 + 15.999 = 18.015 g/mol. This calculator handles single-element lookups; combining them into a compound's molar mass is a short manual sum from there.