Classification of Nuclides:

A nuclide is denoted as ^A_ZX where X is the chemical symbol for the element; the superscript A denotes the mass number and the subscript Z the atomic number of the nuclide.  It will have (A - Z) number of neutrons (N) which are represented on the right side as subscript ^A_ZX _N, if need be.  The value of Z is often omitted because it is uniquely identified by the symbol of the element.

According to stability, all nuclides may be classified into five groups:

i) Stable Nuclides do not undergo radioactive decay and are found in nature. There are about 275 such nuclides to date but sophisticated methods continue to show feeble activity for some and the number keeps decreasing.  For example ¹²C, ¹⁴N, ¹⁶O etc

ii) Primary Natural Radionuclides are found in nature since the origin of the solar system.  These have long half lives, ²³⁸U (t_1/2 = 4.5×10⁹ y), 40K (t_1/2 = 1.3×10⁹ y) and ⁸⁷Rb (t_1/2 = 4.5×10⁹ y).

iii) Secondary Natural Radionuclides are found in nature as decay products of primary natural radionuclides.  Obviously their half-lives are short.  For example ²²⁶Ra (t_1/2 = 1600 y) and ²³⁴Th (t_1/2 = 24.1d) both of which are decay products of ²³⁸U.

iv) Induced Natural Radionuclides are being continuously produced by the action of cosmic rays on the earth's atmosphere and hence these are found in nature. Two well known examples are ³H (t_1/2 = 12.3 y) and ¹⁴C (t_1/2 = 5730y) which are used in radioactive dating.

v) Artificial Radionuclides are produced by the bombardment of nuclear particles on any target nuclide in a nuclear reactor or accelerator and hence are not found in nature.  There are more than 2000 such nuclides.