As most you know the measurement of atomic masses has > 200 year old history and atomic masses were deduced on the basis of chemical reactions by meticulous chemists until the 1940s. Later, mass spectrometer became the tool for determining atomic masses. In each case, scientists assigned one of the elements an arbitrary number and calculated the masses relative to that. For example, early chemists like Dalton, set H as exactly 1. Later, O-16 was set to 16 exactly. By the 1960s, C-12 was chosen as the reference and its atomic mass set to 12. Today, all atomic masses in the periodic table are relative atomic masses with C-12 as a reference.

What puzzles me is that if all atomic masses are relative to one particular element, what is the actual mass of an element? All these definitions were chosen by chemists without any theoretical justification in the past. I mean O-16 was set to 16 when the concept of atomic structure was not even clear.

With the experimental availability of Kibble balances and perfect silicon spheres, would it be possible to get rid of relative atomic masses in future and find the true (absolute) atomic masses of elements?

P.S. Edit The mass of carbon or oxygen atoms originates from a circular definition (as posted in the answer). All these masses are calculated on the basis of assuming that 1 mole of C-12 atoms weighs 12 g exactly. There is a long long history behind it. I didn't want to repeat all here. I am not worried about kg or anything else. All I am saying is that would new approaches like the Kibble balance allow us to get rid of relative atomic masses?

  • $\begingroup$ You define that one mole of C12 is 12 grams. Depending on what version of SI units you look at, this the defines other relationships. There is no mass standard from Mother Nature - it is a human construct. $\endgroup$ – Jon Custer Apr 15 '20 at 13:12
  • $\begingroup$ You may find this work interesting: doi.org/10.1103/PhysRevLett.119.033001 Here they determine the mass of a proton in amu by comparing it to the mass of a six-times-ionized carbon atom (just the nucleus!). This allows a direct comparison without translating amu into kilograms and back. $\endgroup$ – Will C Apr 19 '20 at 4:15

A quick search in google will give you the mass of a carbon atom as $1.9944235\cdot10^{-26\ }kg$ and that of oxygen atom as $2.6566962\cdot10^{-26\ }kg$.

These are absolute masses of the elements in SI units. But, if you think about it, is it any intuitive to use these masses for calculations?

The current atomic mass unit is extremely convenient, and (though I don't have much of an idea about it) I think that physical chemistry has to do a lot with concepts of moles and molar masses, which are rendered far easier and logical by using the relative masses.

By the way, your idea of a theoretical justification is faulty. We have as much theoretical justification for using kilograms as the SI unit as we have for using atomic mass. A kilogram was just the mass of a platinum-iridium cylinder (IPK), and is now defined using Dimensional analysis (if I remember right) after fixing values of second (s), metre (m) and Planck's constant.

In short, almost every one of our measurement is relative, and we use the one most convenient for a particular situation

  • $\begingroup$ The masses that you found from Google are obtained from circular definition. This assumes that 1 mole of C-12 weighs 12 grams exactly. $\endgroup$ – M. Farooq Apr 15 '20 at 5:00
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    $\begingroup$ @M.Farooq Oh, Sorry, I thought that it was measured by using a mass spectrometer. $\endgroup$ – Elendil Apr 15 '20 at 6:00
  • $\begingroup$ Right Krishna, mass spectrometers are calibrated with known chemical compounds whose formula masses are known before hand. $\endgroup$ – M. Farooq Apr 15 '20 at 6:26

The integer numbers come from the experimental observation that atoms are composed out of positively charged nuclei with negative electrons bound around them, and the number of electrons neutralizes the number of protons in the nucleus. So each atom is accorded an Atomic number, which is an integer and characterizes it in the table.

What you call atomic mass units the wikipedia has as Dalton units. In honor of Dalton who first recorded the periodic table which is now ordered according to the atomic number of the elements (i.e. the number of charges ).

The dalton or unified atomic mass unit (symbols: Da or u) is a unit of mass widely used in physics and chemistry. It is defined as 1/12 of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest

It is useful in order for people doing calculations not to have to carry kilograms or other mass units in the microcosm of atoms and molecules.

  • $\begingroup$ I am sorry if you missed my point or I wasn't clear. Indeed O-16 or C-12 indicates the mass number but that does not mean that 1 mole of O-16 that weighs 16 grams exactly (mass defect and so on). $\endgroup$ – M. Farooq Apr 15 '20 at 5:08
  • $\begingroup$ @M.Farooq But that is what I am saying, when you want actual masses you go to dalton units which are the measured masses for each isotope. The integers are atomic numbers characterizing all the isotopes of nuclei with that number of protons. $\endgroup$ – anna v Apr 15 '20 at 5:56
  • $\begingroup$ The integers are the number of positive charges in the nucleus. Maybe this will help sciencedirect.com/topics/engineering/atomic-mass-unit $\endgroup$ – anna v Apr 15 '20 at 6:01
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    $\begingroup$ Yes, anna, as you wrote for "actual masses you go to dalton units which are the measured masses for each isotope", I completely agree, but these measured masses are quoted with reference to $and$ with the assumption that C-12 weighs 12 daltons exactly, if you wish to say it that way. I am a research chemist by profession so I do understand these basic definitions. Sorry if I sounded like a high school student. I have been reading the atomic weight history of 200 years from various sources. My key question is can we get rid of C-12 reference? $\endgroup$ – M. Farooq Apr 15 '20 at 6:25
  • $\begingroup$ @M.Farooq The mass is a variabe, as much as (x,y,z) . For space, we need an to define the unit of length. . Maybe this will help? en.wikipedia.org/wiki/MKS_system_of_units , the meter kilogram second units. They used to have the standard kilogram in Paris in a vault en.wikipedia.org/wiki/Kilogram#Timeline_of_previous_definitions . One then can change the units as long as there is a consistent way to do it. So yes one could take another nucleus. $\endgroup$ – anna v Apr 15 '20 at 8:32

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