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

Question

Wyatt Atkins · Accepted Answer

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.

Kai Bowman · Answer

undefined An element's atomic number is always a round number because it represents the number of protons in the nucleus of an atom, which is an integer value corresponding to the element's identity. Atomic mass, on the other hand, often includes decimal values because it is the weighted average mass of all the isotopes of an element, taking into account their abundance in nature. Isotopes are atoms of the same element with different numbers of neutrons in their nuclei, leading to variations in atomic mass.