Nuclide - a name referring to a given configuration of protons and neutrons arranged in a specific order in an atomic nucleus, in an atom. A given nuclide is also referred to as atomic species.¹⁾ In a wider sense “nuclide” also includes the electrons orbiting the nucleus.²⁾

Helium nuclide: blue - electron orbitals, red - protons, grey - neutrons³⁾⁴⁾

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

Nucleon is a generic name for a proton or neutron. For example, a nucleus of helium contains four nucleons: two protons and two neutrons.

The protons and neutrons in a nucleus can be at different energy states and spins, in some sense similar as electrons on their subshells. The name for such a specific configuration of order and energy is a nuclide. Other names, such as isotope have narrower meanings, as explained below.

Around 3000 nuclides are known to exist or were produced artificially⁵⁾, however some of them have very short life (even less than 1 μs, see the chart below).⁶⁾

Around 272 stable and 55 radioactive nuclides occur naturally on Earth. The others were made artificially.⁷⁾

Isotope is a nuclide with the same number of protons (Z, atomic number), but different number of neutrons, hence also different mass number A. Isotopes exhibit the same chemical properties.

For example, carbon has 15 isotopes⁸⁾, of which ¹²₆C is stable and making up 99% of carbon on Earth, and ¹⁴₆C has half-life of 5730 years and is used for carbon dating.

Isotone is a nuclide with the same number of neutrons but different number of protons. Isotones have different chemical properties.

Example of isotones with two neutrons are: ³₁H (tritium, unstable isotope of hydrogen, one proton) and ⁴₂He (the main stable isotope of helium, two protons).

Another examples of isotones, with 79 neutrons⁹⁾ are: ¹³⁴₅₅Cs, ¹³³₅₄Xe, and ¹³²₅₃I.

Stable nuclide has a configuration which does not naturally transform to a different state, for example through radioactivity. They are expected to have a longevity comparable with the age of our universe.¹⁰⁾

Chart of stable and radioactive nuclides: Z - number of protons (atomic number), N - number of neutrons^{11) by Ben RG, Public domain}

Radionuclide is a nuclide that is unstable and decays by emission of subatomic particles or electromagnetic radiation, or by spontaneous fission.¹²⁾

For example, uranium-238 decays to form thorium-234, by emitting an alpha particle (same as helium nucleus, containing two protons and two neutrons).

Isobar is a nuclide with the same number of nucleons. The combination of protons and neurons can be different, but mass number A (the total sum of neutrons and protons) is the same.¹³⁾

For example, isobars with mass number 82 are: ⁸²Y, ⁸²Sr, ⁸²Rb and ⁸²Kr.

Isomer is a nuclide with the same number of nucleons (similar to isobar, same number of protons and neutrons), but having different energy states and spins.¹⁴⁾

Examples of isomers of the same nuclide are: ⁹⁹Tc and ^99mTc (m denoting a metastable state, see below).

Individual nuclides typically exist in a “ground” energy state (lowest). They can be excited above the ground state, and these excited states are called the isomeric states.¹⁵⁾

The stability of isomeric states can vary from picoseconds to years, with the long-lived being referred to as the metastable states. These states are denoted by m as in ^99mTc.

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