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'''Isotopes''' are two or more [[atom]]s that have the same number of [[proton]]s (i.e. they are of the same [[element]]), but different numbers of [[neutron]]s.<ref>Wile, Dr. Jay L. ''Exploring Creation With Physical Science''. Apologia Educational Ministries, Inc. 1999, 2000</ref> For example, the most common isotope of [[hydrogen]] has a single proton and no neutrons in its [[nucleus]]. A very small proportion of hydrogen atoms are of the isotope deuterium (with one proton and one neutron)<ref>[http://www.purchon.com/chemistry/deuterium.htm Deuterium - Purchon.com]</ref> or tritium (with one proton and two neutrons).<ref>[http://www.physics.isu.edu/radinf/tritium.htm Tritium - Physics.isu.edu]</ref> Another example is the element carbon that most stably occurs in nature as carbon-12, which contains 6 neutrons, indicating an [[atomic mass]] of 12 (6 protons and 6 neutrons). However, an common unstable isotope, carbon-14, which contains 8 neutrons and is popularly used by scientist to [[carbon dating|date]] artifacts, has an atomic mass of 14 (6 protons and 8 neutrons). 

Isotopes can be very difficult to separate because they have the same external electron shell configuration, which gives them basically identical chemical beheavior. This means that they must be separated by physical means such as diffusion, centrifugation, or sometimes fractional distillation. (See also: [[Radioactive isotope]])

==References==
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[[Category:Chemistry]]
[[Category:Physics]]