'''Isotopes''' are two or more [[atom]]s that have the same number of [[proton]]s (ie they are from 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>.