An isotope is one of two or more species of atoms of a chemical element with the same atomic number and position in the periodic table and nearly identical chemical behaviour but with different atomic masses and physical properties.
Before the early 1900s it was generally assumed that the mass of a standard number of atoms of any given element was a basic characteristic of the element. It was also thought that all the atoms of an element were the same and, in particular, had the same mass. The first evidence that two substances with the same chemical properties did not have to be physically identical came from the study of the radioactivity of the heavy elements. Between 1906 and 1907 several investigators showed that ionium (a decay product of uranium) and radiothorium (a decay product of thorium), when mixed with thorium, could not be separated from it by any chemical means. The two substances had radioactive properties quite different from those of thorium and could be shown to have atomic masses differing by several units from that of thorium. The term isotope was introduced in 1913 by the English chemist Frederick Soddy to cover such situations. Not long after the acceptance of these ideas as applied to the heavy elements came indications that isotopy might exist in the main group of naturally occurring stable elements. In 1919 F.W. Aston of England showed conclusively that neon consisted chiefly of two atomic species. This success was followed by the discovery that chlorine had two isotopes. It soon became clear that most elements consisted of a mixture of isotopes, each with an atomic mass close to an integer on the atomic mass scale.
The majority of elements, as found in the Earth's crust and atmosphere, are now known to be mixtures of several isotopes. Such mixtures are in almost unvarying proportions. In its natural form, tin, for example, has 10 isotopes whose atomic masses range from values approximating 112 to 124, but any given sample of the element has an average value of 118.69. In effect, ordinary tin is nature's standardized blend of these 10 atomic species.
In most cases, only the stable isotopes of elements can be found in nature. The unstable, or radioactive, forms decay (break down) spontaneously into entirely different elements at characteristic rates because their ratio of neutrons to protons is either too low or too high for stability. Isotopes of all the elements heavier than bismuth are radioactive. Some of these, such as uranium, do occur naturally because their isotopes have long half-lives.
Excerpt from the Encyclopedia Britannica without permission.