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An isotope is a variant on a basic element, a substance made of atoms with a different number of neutrons than is typical. Except for hydrogen, every atomic nucleus in normal matter is made of both protons and neutrons; the only question is how many of each there are. Typically, the number of protons and neutrons is the same. In an isotope, this balance is frequently broken. For example,²³⁸U, the most common state of uranium, has three more neutrons than ²³⁵U, the form used in nuclear weapons.

A lack of necessary neutrons makes a nucleus unstable. Protons in the nucleus are positively charged, meaning they repel each other. The presence of neutrons is necessary to separate these protons slightly, making the configuration stable. When the configuration is unstable, nuclear decay can result, turning the atoms into showers of radioactive particles.

The rate at which the isotope decays is given by its half-life, the interval after which half of the material breaks down. Half-life varies between a fraction of a second and many times longer than the age of the universe. Some isotopes, like Helium-3, are not radioactive.

Besides the occasional tendency to be radioactive, an isotope tends to behave similarly to the natural element it is a variant on. Isotopes can participate in all the same reactions and have approximately the same mass, except in rare cases. Sometimes, their increased mass means that chemical reactions in which they participate are slightly slower than usual.

There are many hundreds of known isotopes, and probably hundreds more that we haven’t discovered or produced yet. The most famous isotope is ²³⁵U, because of its use in nuclear energy and weaponry. “Enriched” uranium is uranium with a higher concentration of this isotope, while “depleted” uranium has a much lower concentration. An isotope that is only slightly radioactive can be used as an atomic marker in medical applications, for example to track the spread of a drug in the body.

Carbon-14, which comprises a trace amount of all carbon on earth, is a radioactive isotope with concentration levels that can be used to determine the precise age of an artifact or fossil. Helium-3, thought to exist in large quantities on the moon, is among the most promising long-term fuels for fusion power reactors. However, utilizing it effectively will require first mastering other forms of fusion.

(Source: http://www.wisegeek.com/what-is-an-isotope.htm)