Nuclear isomer in the context of Nuclear clock

Aluminium or aluminum (₁₃Al) has one stable isotope, Al, comprising all natural aluminium. The radioactive Al, with half-life 717,000 years, occurs in traces from cosmic-ray spallation of argon in the atmosphere.

Other than Al, there are 22 known synthetic radioisotopes from Al to Al, and 4 known metastable states; all have half-lives under 7 minutes, most under a second.

Thorium (₉₀Th) has seven naturally occurring isotopes but none are stable. One isotope, Th, is relatively stable, with a half-life of 1.40×10 years, considerably longer than the age of the Earth, and even slightly longer than the generally accepted age of the universe. This isotope makes up nearly all natural thorium, so thorium was considered to be mononuclidic. However, in 2013, IUPAC reclassified thorium as binuclidic, due to large amounts of Th in deep seawater. Thorium has a characteristic terrestrial isotopic composition and thus a standard atomic weight can be given.

Thirty-one radioisotopes have been characterized, with the most stable being Th, Th with a half-life of 75,400 years, Th with a half-life of 7,916 years, and Th with a half-life of 1.91 years. All of the remaining radioactive isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes. One isotope, Th, has a nuclear isomer (or metastable state) with a remarkably low excitation energy, recently measured to be 8.355733554021(8) eV It has been proposed to perform laser spectroscopy of the Th nucleus and use the low-energy transition for the development of a nuclear clock of extremely high accuracy.

Natural nitrogen (₇N) consists of two stable isotopes: the vast majority (99.62%) of naturally occurring nitrogen is nitrogen-14, with the remainder (0.38%) being nitrogen-15. Thirteen radioisotopes are also known, with atomic masses ranging from 9 to 23, along with three nuclear isomers. All of these radioisotopes are short-lived, the longest-lived being N with a half-life of 9.965 minutes. All of the others have half-lives shorter than ten seconds. Isotopes lighter than the stable ones generally decay to isotopes of carbon, and those heavier beta decay to isotopes of oxygen.

Nitrogen-13 is a positron emitter and one of the main isotopes used in medical PET scans.

Naturally occurring zirconium (₄₀Zr) is composed of four stable isotopes (one, Zr, may in the future be found radioactive), and one very long-lived radioisotope (Zr), a primordial nuclide that decays via double beta decay with an observed half-life of 2.34 × 10 years; it can also undergo single beta decay, which is not yet observed, but the theoretically predicted value of t_1/2 is 2.4 × 10 years. The second most stable radioisotope is Zr, which has a half-life of 1.61 million years. Thirty other radioisotopes have been observed from Zr to Zr; all have half-lives less than a day except for Zr (64.032 days), Zr (83.4 days), and Zr (78.36 hours). The most stable of the isomeric states is just 4.16 minutes for Zr.

Radioactive isotopes above the theoretically stable mass numbers 90-92 decay by electron emission resulting in niobium isotopes, whereas those below by positron emission or electron capture, resulting in yttrium isotopes.

Bismuth (₈₃Bi) has 41 known isotopes, ranging from Bi to Bi. Bismuth has no stable isotopes, but does have one naturally occurring, very long-lived isotope; thus, the standard atomic weight can be given from that isotope, bismuth-209. Though it is now known to be radioactive, it may still be considered practically stable because it has a half-life of 2.01×10 years, which is more than a billion times the age of the universe.

Besides Bi, the most stable bismuth radioisotopes are Bi with a half-life of 3.04 million years, Bi with a half-life of 368,000 years and Bi, with a half-life of 31.22 years, none of which occur in nature. All other isotopes have half-lives under 15 days, most under two hours. Of naturally occurring radioisotopes, the most stable is radiogenic Bi with a half-life of 5.012 days. Bi is unusual for being a nuclear isomer with a half-life many orders of magnitude longer than that of the ground state.

Natural gallium (₃₁Ga) consists of a mixture of two stable isotopes: gallium-69 and gallium-71. Synthetic radioisotopes are known with atomic masses ranging from 60 to 89, along with seven nuclear isomers. Most of the isotopes with atomic mass numbers below 69 decay by electron capture and positron emission to isotopes of zinc, while most of the isotopes with masses above 71 beta decay to isotopes of germanium.

The medically important radioisotopes are gallium-67 and gallium-68, used for imaging, and further described below.

There are 21 known isotopes of sodium (₁₁Na), ranging from
Na to
Na (except for Na and Na), and five isomers.
Na is the only stable (and the only primordial) isotope, making sodium a monoisotopic (and mononuclidic) element. Sodium has two radioactive cosmogenic isotopes (
Na, with a half-life of 2.6019 years and
Na, with a half-life of 14.956 hours). With the exception of those two isotopes, all other isotopes have half-lives under a minute, most under a second.

Acute neutron radiation exposure (e.g., from a nuclear criticality accident) converts some of the stable
Na in human blood plasma to
Na. The neutron radiation dose absorbed by the patient can be assessed by measuring the concentration of the radioisotope.

Beryllium (₄Be) has 11 known isotopes and 3 known isomers, but only one of these isotopes (
Be) is stable and a primordial nuclide. As such, beryllium is considered a monoisotopic element. It is also a mononuclidic element, because its other isotopes have such short half-lives that none are primordial and their abundance is very low. Beryllium is unique as being the only monoisotopic element with an even number of protons (even atomic number) and also has an odd number of neutrons; the 25 other monoisotopic elements all have odd numbers of protons (odd atomic number), and even of neutrons, so the total mass number is still odd.

Of the 10 radioisotopes of beryllium, the most stable are
Be with a half-life of 1.387 million years and
Be with a half-life of 53.22 days. All other radioisotopes have half-lives shorter than 15 seconds.

Uranium-234 (
U or U-234) is an isotope of uranium. In natural uranium and in uranium ore, U occurs as an indirect decay product of uranium-238, but it makes up only 0.0055% (55 parts per million, or 1/18,000) of the raw uranium because its half-life of just 245,500 years is only about 1/18,000 as long as that of U. Thus the ratio of
U to
U in a natural sample is equivalent to the ratio of their half-lives. The primary path of production of U via nuclear decay is as follows: uranium-238 nuclei emit an alpha particle to become thorium-234. Next, with a short half-life, Th nuclei emit a beta particle to become protactinium-234 (Pa or more usually the isomer Pa). Finally, Pa or Pa nuclei emit another beta particle to become U nuclei.

Uranium-234 nuclei decay by alpha emission to thorium-230, except for the tiny fraction (here less than 2 per trillion) of nuclei that undergo spontaneous fission.