Isotopes are two forms of an element with the same atomic number but different mass number. The existence of isotopes can be understood by reviewing the structure of atoms.
All atoms contain three kinds of basic particles: protons, neutrons, and electrons. (Hydrogen is the only exception to this statement; most hydrogen atoms contain no neutrons.) The protons and neutrons in an atom are found in the atomic nucleus, while the electrons are found in the space around the nucleus.
The number of protons in a nucleus defines an atom. Hydrogen atoms all have one proton in their nucleus; helium atoms all have two protons in their nucleus; lithium atoms all have three protons in their nucleus; and so on. The number of protons in an atom's nucleus is called its atomic number. Hydrogen has an atomic number of 1; helium, an atomic number of 2; and lithium, an atomic number of 3.
But atoms of the same element can have different numbers of neutrons. Some helium nuclei, for example, have two neutrons; others have only one. The mass number of an atom is the total number of protons and neutrons in the atom's nucleus. The two-neutron atom of helium has a mass number of four (two protons plus two neutrons). The one-neutron atom of helium has a mass number of three (two protons plus one neutron).
Another way of defining isotopes, then, is to say that they are different forms of an atom with the same number of protons but different numbers of neutrons.
Most elements have at least two stable isotopes. The term stable here means not radioactive. Twenty elements, including fluorine, sodium, aluminum, phosphorus, and gold, have only one stable isotope. By contrast, tin has the largest number of stable isotopes of any element, ten.
Isotopes are commonly represented in one of two ways. First, they may be designated by writing the name of the element followed by the mass number of the isotope. The two forms of helium are called helium-4 and helium-3. Second, isotopes may be designated by the chemical symbol of the element with a superscript that shows their mass number. The designations for the two isotopes of helium are 4 He and 3 He.
A radioactive isotope is an isotope that spontaneously breaks apart, changing into some other isotope. As an example, potassium has a radioactive isotope with mass number 40, 40 K or potassium-40. This isotope breaks down into a stable isotope of potassium, 39 K or potassium-39.
Radioactive isotopes are much more common than are stable isotopes. At least 1,000 radioactive isotopes occur in nature or have been produced synthetically in particle accelerators (atom-smashers) or nuclear reactors (devices used to control the release of energy from nuclear reactions).
Isotopes have many important applications in theoretical and practical research. The advantage of using two or more isotopes of the same element is that the isotopes will all have the same chemical properties but may differ from each other because of their mass differences. This difference allows scientists to separate one isotope from another. An important example of this process is the way isotopes were used to purify uranium during World War II (1939–45).
Two common isotopes of uranium exist, 235 U and 238 U. Of these two, 238 U is much more abundant, making up about 99.3 percent of the uranium found in nature. But only 235 U can be used in making nuclear weapons and nuclear reactors. Since both 235 U and 238 U have the same chemical properties, how can the valuable 235 U be separated from the more abundant, but valueless, 238 U?
One answer to this question was to convert natural uranium to a gas and then allow the gas to diffuse (spread out) through a porous barrier. Researchers found that the 235 U in the natural uranium was slightly less heavy than the 238 U, so it diffused through the barrier slightly more quickly. But because the difference in mass between the two isotopes is not very great, the diffusion had to be repeated many times before the two isotopes could be separated very well. Eventually, however, enough 235 U was collected by this process to make the world's first nuclear weapons.