Atomic mass lookup
Look up any element by symbol, name or atomic number. You get the standard atomic weight, its uncertainty, and — where there is one — the reason the element refuses to have a single value. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
Fourteen elements have no single atomic weight
A periodic table on a classroom wall gives lead as 207.2. The full CIAAW table gives it as [206.14, 207.94] — an interval, not a number.
That is not caution about measurement. It is a statement about lead. Lead is the end point of three separate radioactive decay chains, so the isotopic mix in a piece of lead depends on what the ore had in it and how long ago. Lead from a uranium-rich ore is genuinely heavier than lead from a thorium-rich one. There is no single number because there is no single lead.
The fourteen are hydrogen, lithium, boron, carbon, nitrogen, oxygen, magnesium, silicon, sulfur, chlorine, argon, bromine, thallium and lead.
Argon is the most extreme. [39.792, 39.963]. The top of that range is the atomic mass of argon-40 itself, because some samples are very nearly pure radiogenic argon-40 — potassium decays to it, and it accumulates. The bottom comes from pitchblende, where nuclear reactions have made unusual amounts of the lighter isotopes. The familiar 39.948 is specifically argon separated from air, and air is only one place argon comes from.
What the g, m and r mean
| g | Geological and biological materials are known whose isotopic composition lies outside the range for normal material. The atomic weight in those can differ by more than the stated uncertainty. |
|---|---|
| m | Commercially available material may have been through undisclosed or accidental isotopic fractionation, and can differ substantially. Lithium is the classic case. |
| r | The natural range of isotopic composition is itself what stops a more precise value being given. |
The thirty-four with no value at all
Technetium, promethium and everything past bismuth except thorium, protactinium and uranium have no standard atomic weight, because they have no isotope stable enough for the idea to mean anything. This page says so rather than printing something.
Tables that do show a figure for technetium are almost always showing [98] — the mass number of its longest-lived isotope. That is a count of nucleons, not a weighted average of what you would find in a rock, and the square brackets are the only thing distinguishing it from a quantity of a completely different kind.
Where these numbers come from
The atomic weights on this page are the abridged standard atomic weights 2024, © IUPAC / CIAAW — the Commission on Isotopic Abundances and Atomic Weights of the International Union of Pure and Applied Chemistry, published at ciaaw.org and reproduced under their terms. CIAAW has published the critical evaluation of atomic weights since 1902.
The abridged table is the one used here rather than the full one, and that is a decision worth stating. Fourteen elements have no single atomic weight in the full table at all — they get an interval instead — and taking the middle of an interval would be this site inventing a number nobody published. CIAAW publish the abridged table with a single value and an uncertainty for exactly this purpose, and that is what you see.