Vitamin B12, also known as cyanocobalamin, is a water-soluble vitamin that is part of the B vitamin family. B vitamins help support adrenal function, help calm and maintain a healthy nervous system, and are necessary for key metabolic processes.
For each of the 80 stable elements, the number of the stable isotopes is given. Only 90 isotopes are expected to be perfectly stable, and an additional 163 are energetically unstable, but have never been observed to decay. Thus, 253 isotopes (nuclides) are stable by definition (including tantalum-180m, for which no decay has yet been observed).
In the case of iodine-131 (131 I), the atomic number Z (number of protons) is 53 and number of neutrons is 78. So the total mass number is 131. An unstable nuclide tries to attain stability through radioactive decay releasing alpha, beta and gamma rays. In medicine, this radiation is used for diagnosis and treatment.