Most of those are only produced artificially, and have very short half-lives.
For comparison, there are about 254 stable nuclides. Even the lightest element, hydrogen, has a well-known radionuclide, tritium.
Trees undergo spurts in growth in the spring and summer months while becoming somewhat dormant in the fall and winter months.
Over 60 further radionuclides are detectable in nature, either as daughters of these, or through natural production on Earth by cosmic radiation.
More than 2400 radionuclides have half-lives less than 60 minutes.
A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is an atom that has excess nuclear energy, making it unstable.
This excess energy can be either emitted from the nucleus as gamma radiation, or create and emit from the nucleus a new particle (alpha particle or beta particle), or transfer this excess energy to one of its electrons, causing that electron to be ejected as a conversion electron.
The degree of harm will depend on the nature and extent of the radiation produced, the amount and nature of exposure (close contact, inhalation or ingestion), and the biochemical properties of the element; with increased risk of cancer the most usual consequence.
However, radionuclides with suitable properties are used in nuclear medicine for both diagnosis and treatment.
During those processes, the radionuclide is said to undergo radioactive decay. The unstable nucleus is more stable following the emission, but will sometimes undergo further decay.
Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay.) for that collection can be calculated from their measured decay constants.
The use of 14C in meteorite dating is solely based on its production by cosmic rays (and for terrestrial samples, with its production in the atmosphere).
26Al and some other nuclides not mentioned are also used in this way.
The range of the half-lives of radioactive atoms have no known limits and span a time range of over 55 orders of magnitude.