Relative isotopic mass (a property of a single atom) is not to be confused with the averaged quantity atomic weight (see above), that is an average of values for many atoms in a … See more

The amount that the ratio of atomic masses to mass number deviates from 1 is as follows: the deviation starts positive at hydrogen-1, then decreases until it reaches a local minimum at helium-4. Isotopes of lithium, beryllium, and boron are less strongly … See more

Similar definitions apply to molecules. One can calculate the molecular mass of a compound by adding the atomic masses (not the standard … See more

• Atomic number
• Dalton (unit)
• Isotope
• Isotope geochemistry
• Molecular mass
• Jean Stas See more

The atomic mass or relative isotopic mass are sometimes confused, or incorrectly used, as synonyms of relative atomic mass (also known as atomic weight) or the standard atomic weight (a particular variety of atomic weight, in the sense that it is standardized). … See more

Direct comparison and measurement of the masses of atoms is achieved with mass spectrometry. See more

The first scientists to determine relative atomic masses were John Dalton and Thomas Thomson between 1803 and 1805 and Jöns Jakob Berzelius between 1808 and 1826. Relative … See more