Why do francium, radon, radium, and actinium have whole numbers for average atomic mass even though they have isotopes? But then elements like beryllium, fluorine, and sodium are listed as having no isotopes but yet they have a decimal for their average atomic mass?

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    $\begingroup$ The Wikipedia data for Francium, Radon, Radium, and Actinium has mass numbers (protons + neutrons) not atomic masses since all the isotopes of these elements are radioactive. $\endgroup$ – MaxW Nov 1 '18 at 19:50
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    $\begingroup$ Elements' names aren't capitalised. $\endgroup$ – Mithoron Nov 1 '18 at 21:38
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    $\begingroup$ It's not what's going on here, but note that it's perfectly possible for the weighted average of a set of integers to be an integer. $\endgroup$ – David Richerby Nov 2 '18 at 14:08

For elements with no stable isotope (i.e. Francium, Radium, and Actinium), the atomic mass is chosen to be that of the longest lived isotope.

More generally, the masses for stable elements are reflective of the natural abundance of each isotope in a sample of the element. Sodium has more than one isotope, so the statement is not really true, though only one of them is stable and included in the averaging. In this case, it's important to remember that the mass of protons and neutrons are only approximately 1 amu, so even if there is only one isotope there's no requirement that it have an integer atomic mass.


For highly radioactive elements that don't occur in large quantities in nature, the mass number listed is the mass number of the most stable isotope. When this is done, the value is often written in brackets. The wikipedia page for the Periodic table has some nice details on the organization of the table and the values included.


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