I am confused about the definition of isotopes I know that isotopes are atoms of an element with different numbers of neutrons. What I don't understand is that we have monoisotopic elements which are elements that have only a single stable isotope.

Why do we say isotope in this sentence, we are not comparing the atoms of that element with each other (for example we can not say the number of neutrons is different cause there is only one type of atom)?

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    $\begingroup$ Unknow what you know. Isotopes are atoms of an element with the same numbers of neutrons. $\endgroup$ Oct 21, 2022 at 10:33
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    $\begingroup$ Always try to find answers on your own in offline and online resources, before asking. It gives you more than when just receiving the answer. $\endgroup$
    – Poutnik
    Oct 21, 2022 at 10:34
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    $\begingroup$ There are many available resources for the topic of isotopes. Multiple Wikipedia pages as the comprehensible summary and source of primary references. $\endgroup$
    – Poutnik
    Oct 21, 2022 at 11:10
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    $\begingroup$ This post could use a more descriptive title. $\endgroup$
    – Buck Thorn
    Oct 21, 2022 at 15:12
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    $\begingroup$ You were taught a sloppy definition of "isotope. ;) An isotope is a type of atom that has a defined number of protons AND neutrons. $\endgroup$
    – Karl
    Oct 21, 2022 at 17:09

1 Answer 1


An isotope of the given element is an atom (or element consisting of just such atoms) with the particular neutron or nucleon number.

Note that every element has more than one isotope, often dozens of isotopes, but many, most or all are unstable. Search for Wikipedia articles with names "Isotopes of <element>"

In this context, elements with odd proton number have maximally 2 stable isotopes, sometimes just one like fluorine or none like technetium.

E.g. fluorine has 18 known isotopes.

See also

  • 26 monoisotopic elements which have only a single stable isotope (nuclide).
  • 21 mononuclidic elements, found naturally on Earth essentially as a single nuclide (which may, or may not, be a stable nuclide).

You may notice also the term nuclide with slightly different meaning than an isotope, and used in a different context. Generally, the term isotope is used in context of chemical or physical atomic properties, while nuclides in context of nuclear properties. Differences arise if a nucleus is known to exist in different states ("nuclear isomers") with different energy, spin, stability and possibly decay mode schema.


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