Isotopes

Atoms with similar atomic number and dissimilar numbers of neutrons are known as isotopes. The element's chemical properties are determined largely by the charge on the nucleus, and dissimilar isotopes of an element have very identical chemical properties. Though, they are not quite similar, and slight variations in chemistry and in physical properties allow isotopes to be separated if desired.

Some elements have only one stable isotope (for example ¹⁹F, ²⁷Al, ³¹P), others may have various (for example ¹H and ²H, the latter also being called deuterium, ¹²C and ¹³C); the record is held by the tin (Sn), which has no fewer than 10. The Natural samples of many elements therefore contain mixtures of isotopes in almost fixed proportions reflecting the ways in which these were made by nuclear synthesis. The molar mass (also termed as relative atomic mass, abbreviated as RAM) of elements is determined by these proportions. For several chemical purposes the presence of such type of isotopic mixtures can be ignored, even though it is occasionally important.

1. Slight variations in chemical and physical properties can lead to small differences in the isotopic composition of natural samples. They can be exploited to give the geological information like dating and origin of rocks, etc. and lead to small differences in the molar mass of elements.
2. Some spectroscopic methods (particularly nuclear magnetic resonance, NMR,) exploit particular properties of specific nuclei. Two significant NMR nuclei are ¹H and ¹³C. The former makes up over 99.9% of natural hydrogen, but ¹³C is exist as only 1.1% of natural carbon. These distinct abundances are significant both for the sensitivity of the method and the appearance of the spectra.
3. Isotopes can be separated and employed for particular purposes. So the slight variations in chemical behavior between normal hydrogen (¹H) and deuterium (²H) can be used to investigate the detailed procedures of chemical reactions involving hydrogen atoms.

All elements have unstable radioactive ones, in addition to stable isotopes. Some of these take place naturally, and in particle accelerators or nuclear reactors the others can be made artificially. Several radioactive isotopes are used in biochemical and chemical research and for medical diagnostics.