Molar MassAtomic weights: where the numbers come from
Every molar mass begins with atomic weights. They look like fixed facts of nature, but they are measured values, reviewed and published by an international committee.
Why they are not whole numbers
Most elements are mixtures of isotopes: atoms with the same number of protons but different numbers of neutrons, and so different masses. Chlorine is roughly three quarters chlorine-35 and one quarter chlorine-37. The atomic weight is the weighted average, about 35.45. Nobody has ever held an atom of chlorine with that mass. It is the average for chlorine as found on Earth.
Even an element with a single natural isotope does not come out whole. Fluorine is 18.998, not 19, because the mass of a nucleus is slightly less than the sum of its separate protons and neutrons, and because the scale is set by carbon-12, not by the proton.
Who decides
The values are published by the Commission on Isotopic Abundances and Atomic Weights, known as CIAAW, a body of the International Union of Pure and Applied Chemistry, IUPAC. It reviews new measurements every two years and revises a value when the evidence justifies it. The table used on this site is CIAAW's abridged table, in which each value is cut to five significant figures. That is more than enough for school, college and most laboratory work.
Why some values carry a plus or minus
The isotope mixture of some elements differs slightly from one natural source to another. Hydrogen in sea water is not quite the same mixture as hydrogen in natural gas. For such elements, including hydrogen, lithium, boron, carbon, nitrogen, oxygen, sulphur and chlorine, CIAAW gives a range for the full value and a single abridged value with an uncertainty. Lithium is the extreme case: 6.94 ± 0.06.
This is why a careful molar mass should not be quoted to too many digits. Writing the molar mass of water as 18.01528 claims more than nature allows. 18.015 or 18.02 is honest.
Elements with no standard atomic weight
Elements that have no stable isotopes and are not found in nature in a steady mixture have no standard atomic weight. Technetium, promethium and all elements after uranium are examples. Tables often show the mass number of their longest-lived isotope in square brackets. The calculator on this site will tell you when a formula contains one of them rather than guess.
School values and full values
| Element | Usual school value | IUPAC abridged value |
|---|---|---|
| Hydrogen | 1 | 1.0080 |
| Carbon | 12 | 12.011 |
| Nitrogen | 14 | 14.007 |
| Oxygen | 16 | 15.999 |
| Sodium | 23 | 22.990 |
| Sulphur | 32 | 32.06 |
| Chlorine | 35.5 | 35.45 |
| Calcium | 40 | 40.078 |
| Iron | 56 | 55.845 |
| Copper | 63.5 | 63.546 |
The differences are small, usually well under 1 percent. In an exam, use the values given in the paper. For laboratory work, use the fuller ones.
Atomic weight, atomic mass, mass number
- Mass number: protons plus neutrons in one atom. Always a whole number. Carbon-12 has mass number 12.
- Atomic mass: the mass of one particular atom, in unified atomic mass units, u.
- Atomic weight, or relative atomic mass: the average for the natural mixture, with no unit. This is the number in the periodic table.
The full list used by this site is on the atomic weights table. For everything else about an element, see the periodic table site.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.