Why is an element's atomic number always a round number, whereas its atomic mass often is not?

1 Answer
Sep 3, 2016


Because the element is defined by its #"atomic number, Z......."#


Because the element is defined by its #"atomic number, Z"#, which is the number of positively charged, massive particles in the element's nucleus.

If #Z=1#, the element is hydrogen, if #Z=2#, the element is helium,.......if #Z=34#, the element is selenium....

Now an element's identity is thus defined by #Z#. However, the elemental core, the nucleus, may contain different numbers of massive, neutrally charged particles, neutrons, and this gives rise to the phenomenon of isotopes.

For hydrogen, #Z=1#, by definition. Most hydrogen nuclei contain only the 1 proton, to give the #""^1H#, the protium isotope; a smaller number of hydrogen nuclei may contain a neutron in addition to the discriminating proton (why discriminating?) to give the deuterium isotope, #""^2H#, and an even smaller number of hydrogen nuclei may contain 2 neutrons to give the tritium isotope, #""^3H#.

As the atomic number gets larger, the possibility for isotopic stability becomes greater. Many electron atoms (or many proton atoms) tend to have a range of isotopes, whose weighted average defines the quoted average isotopic mass. And thus the average atomic mass is typically not a whole number.

Of course the mass of any particular isotope is necessarily a whole number. Peculiar isotopic properties may be exploited by chemists to give a spectroscopic handle on chemistry. Nuclear magnetic resonance spectroscopy is one example, as there is usually an isotope with useful magnetic properties. Alternatively, isotopes may be used for mass spectroscopic studies - deuterium labelling is a frequent experiment.