What Is Atomic Mass

Faraday's Constant


The answer to ‘what is atomic mass’ is this: the total mass of the protons, neutrons, and electrons in a single atom when it is at rest. This is not to be associated or mistaken for atomic weight. Atomic mass is measured by mass spectrometry. You can figure the molecular mass of an compound by adding the atomic mass of its atoms.

Until the 1960’s chemists and physicists used different atomic mass scales. Chemists used a scale that showed that the natural mixture of oxygen isotopes had an atomic mass 16. Physicists assigned 16 to the atomic mass of the most common oxygen isotope. Problems and inconsistencies arose because oxygen 17 and oxygen 18 are also present in natural oxygen. This created two different tables of atomic mass. A unified scale based on carbon-12 is used to meet the physicists’ need to base the scale on a pure isotope and is numerically close to the chemists’ scale.

Standard atomic weight is the average relative atomic mass of an element in the crust of Earth and its atmosphere. This is what is included in standard periodic tables. Atomic weight is being phased out slowly and being replaced by relative atomic mass. This shift in wording dates back to the 1960’s. It has been the source of much debate largely surrounding the adoption of the unified atomic mass unit and the realization that ‘weight’ can be an inappropriate term. Atomic weight is different from atomic mass in that it refers to the most abundant isotope in an element and atomic mass directly addresses a single atom or isotope.

Atomic mass and standard atomic weight can be so close, in elements with a single dominant isotope, that there is little difference when considering bulk calculations. Large variations can occur in elements with many common isotopes. Both have their place in science today. With advances in our knowledge, even these terms may become obsolete in the future.

We have written many articles about atomic mass for Universe Today. Here’s an article about the atomic nucleus, and here’s an article about the atomic models.

If you’d like more info on the Atomic Mass, check out NASA’s Article on Analyzing Tiny Samples, and here’s a link to NASA’s Article about Atoms, Elements, and Isotopes.

We’ve also recorded an entire episode of Astronomy Cast all about the Atom. Listen here, Episode 164: Inside the Atom.

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Atomic Mass

Faraday's Constant

The mass of an atom is its atomic mass (duh!).

Actually, it’s worth looking into this a bit more deeply … it’s not as simple as the “duh!” implies …

An atom is made up of protons (at least one), neutrons (except for hydrogen), and electrons (at least one), so its mass is simply the total of the masses of protons, neutrons, and electrons, right? Wrong … the nucleus of any atom (except hydrogen) is held together by the strong nuclear force, and the electrons are bound to the atom by the electromagnetic force; it takes energy to break up a nucleus, and energy to free an electron from an atom … and mass and energy are related (remember E = mc2?); the stronger the binding, the more the mass of an atom differs from the sum of the masses of its individual components!

Also, there’s atomic weight (atomic mass applies to each isotope of an element; atomic weight is an average, for each element, of the atomic masses of the isotopes … weighted by their relative abundance); relative atomic mass (a synonym for atomic weight, and also – confusingly – the small difference between standard atomic weight and the atomic weight of a particular sample!); and … you get the idea.

Atomic mass is usually measured in atomic mass units (no, no “duh!” this time, as you’ll see), which is defined as 1/12th of the mass of an isolated carbon-12 atom, at rest, in its ground state … and this is the unified atomic mass unit (symbol u), to distinguish it from the older atomic mass unit (amu). Why? Why go to all this trouble? Because there are actually two different amu’s! And both are different from u!! Both are based on oxygen (rather than carbon); one on the oxygen-16 isotope, the other on oxygen, the mixture of isotopes.


More on atomic mass: from NASA Atoms, Elements, and Isotopes; The Mass Spectrometer of the Galileo Probe , and this Lawrence Berkeley National Lab webpage.

Are there any Universe Today stories featuring atomic mass? Sure! Mini-Detector Could Find Life on Mars or Anthrax at the Airport, Super-Neutron Stars are Possible, and Learning to Breathe Mars Air, to give just three examples.

Are there any Astronomy Cast episodes on atomic mass? Sure! Inside the Atom, and Energy Levels and Spectra, to give just two examples.