More than 280 isotopes are known, which occur in nature, out of which 40 are radioactive.
Moreover, about 300 artificial isotopes have been produced through disintegration.
The distribution of isotopes among the elements is varied and complex. For example arsenic, fluorine, iodine and gold have only one isotope. This is why they are called mono-isotopic substances.
In general, the elements having odd atomic number have never more than two stable isotopes, whereas the elements having even atomic number have much larger number of isotopes. Isotopes whose mass number is multiples of four are particularly abundant.
For example: 16O, 24Mg, 28Si, 40Ca and 56Fe form nearly 50% of earth’s crust.
Out of 280 isotopes that occur in nature, 154 are of even mass number and of even atomic number.