The following glossary includes terms that are used in Student Text, Epoch Six in our Evolving Universe.

Isotope
Atoms of a given element have the same number of protons, but atoms of some elements can have different numbers of neutrons. When this occurs, the element is said to have more than one isotope.

Deuterium
Deuterium is the isotope of hydrogen that has one proton and one neutron in the nucleus. About three percent of naturally occurring hydrogen is deuterium. The most abundant isotope of hydrogen has one proton and no neutrons.