I was looking up fluorine isotopes out of curiosity. The Wikipedia entry gives info on them but adds there are also two isomers which are $\mathrm{^{18m}F}$ and $\mathrm{^{26m}F}$. What is an isomer of an atomic nucleus please someone?

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    $\begingroup$ Basically an isomer is a nucleus that is in an excited state and upon relaxation emits a gamma ray. Similar to (in principle) to how electrons get excited in phosphorescence and released hours later. A cool example is Tantalum 180 is radioactive with a half-life of 8 hours. Tantalum 180m is stable. $\endgroup$ – A.K. Apr 26 '19 at 19:03
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    $\begingroup$ Related: How do I correctly typeset metastable radionuclide symbols? $\endgroup$ – Faded Giant Apr 26 '19 at 19:04
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    $\begingroup$ Link to that wiki en.wikipedia.org/wiki/Isotopes_of_fluorine#List_of_isotopes and no, they don't have any chemical relevance. $\endgroup$ – Karl Apr 26 '19 at 20:09
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    $\begingroup$ Disagree with closure but have no votes - if gamma radiation is considered relevant to chemistry so should this $\endgroup$ – Ian Bush Apr 27 '19 at 10:04
  • $\begingroup$ @Karl en.wikipedia.org/wiki/Fluorine-18 $\endgroup$ – Ian Bush Apr 27 '19 at 10:13

Just like electrons nuclei can exist in a number of quantised states. We see evidence of this when, during radioactive decay, a gamma ray is produced. This is a nucleus in an excited state decaying to the ground state, and the excess energy being radiated away as a gamma ray photon. This is precisely analogous to an electron in an atom falling from a high energy state to a lower energy one, and thus producing radiation which we can see in an atomic spectrum. So an isomer of a nucleus is simply that nucleus in a state which is not the ground (lowest energy) state.

Most isomers are very short lived, and thus the gamma radiation appears to be more or less coincident with any other radiation that is produced during a radioactive decay. For instance the $\ce{^{18m}F}$ isomer you mention has a lifetime of 162ns according to https://en.wikipedia.org/wiki/Isotopes_of_fluorine . However the odd one is much longer lived, and for a remarkable case of one that is observationally stable take a look at $\ce{^{180m}Ta}$. 0.012% of natural Tantalum consists of this isomer, and it has never been observed to decay to the ground state - as is usual in such cases symmetry forbids the transition. And what I find even more remarkable is that the ground state itself is unstable with respect to both electron capture and beta emission, decaying to isotopes of Hafnium and Tungsten with a half life of just over 8 hours! See https://en.wikipedia.org/wiki/Isotopes_of_tantalum for details.

Sorry - slightly belated edit, but it occurred to me that isomers do have one very direct use in Chemistry, namely Mossabuer spectroscopy, see https://en.wikipedia.org/wiki/M%C3%B6ssbauer_spectroscopy. In essence very small changes in the energy of the gamma ray that is emitted when the isomer decays to the ground state can be detected by this technique, and can be related to changes in the chemical environment within which the atom finds itself.

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The nucleus of an atom contains a mass of quarks which combine to form a larger particle which is the nucleus. It is possible to assemble these quarks in different ways, for example sometimes the different excited states of a nucleus have different nuclear spins. This is due to the quarks being arranged differently.

Chemistry can be defined as the study of the interaction of atoms with each other and the substances which can formed by doing things with the atoms. We can regard the whole of chemistry as a special area of electrostatics and the study the things which can ´be formed in this way. The whole of chemical bonding can be understood by considering the electrostatic effects between nuclei and electrons. While there is a set of rules which are well understood in chemistry (MO theory) for how the bonding in a molecule works. I am sure that in atomic nuclei that there are rules for the order of the filling of some shells which are similar in concept to the atomic / molecular orbitals.

Now after learning about atomic orbitals and MO theory it is easy to understand the idea of an excited electronic states. For example it is possible to move an electron from the antibonding HOMO of di beryllium into a higher energy orbital which is bonding. Thus allowing us to have diberylium only in an excited state.

Equally if we excite an atomic nucleus we could move quarks from their positions in the ground state of the nucleus into higher energy shells. When this happens the energy of the system is adsorbed by moving the quark into a higher energy shell. The energies of excited states of nuclei clearly are only at well defined energy levels. Consider for a moment the energy levels of a nickel-60, there are three commonly used levels which defines the fact that the excited nickel-60 nuclei from the beta decay of cobalt-60 emit gamma photons at 1.1 and 1.3 MeV. Very occasionally a nucleus jumps from the + 2.4 MeV level to the ground level emitting an extra high energy gamma photon.

One of the biggest bits of evidence for the nuclear shell model is the fact that there are nuclei with magic numbers which have special stability. For example carbon-12 is very stable (but not magic). There is a nuclear reaction which is similar to photoionization of a molecules / atom. This is the photonuclear reaction where the adsorption of a gamma ray causes an atom to eject a neutron. For C-12 to eject a neutron a staggering 18.722 MeV photon would be required. For C-13 to eject a neutron only a 4.946 MeV photon will be needed.

If we take the oxygen isotopes (O-16 is magic), then it is clear that when we are dealing with a magic +1 neutron then the effect is much stronger.

To eject a proton from oxygen nuclei

O-15 requires a 13.22 MeV photon. O-16 requires a 15.66 MeV photon. O-17 requires a 4.14 MeV photon. O-18 requires a 8.04 MeV photon.

This shows that like an alkali metal a nucleus with an extra neutron above one which has both the magic number of protons and neutrons is special. In an alkali metal it is very easy to release that single electron while the energy required to release the extra neutron is lower.

While the question might be formally nuclear physics I do not see a clear line which separates chemistry from physics. Many radioactive decay processes and nuclear reactions do look like something out of chemistry.

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