CoreChem: Isotopes

What are Isotopes

Isotopes are atoms of the same element that have different numbers of neutrons

in the nucleus and therefore different atomic weights

(shown as a superscript numeral in front of the element's symbol)

Isotopes may be stable or radioactive. Most elements have two or more isotopes.

The isotopes most commonly used in geochemistry include: carbon (12C, 13C,14C),

hydrogen (1H, 2H, 3H), oxygen (16O,18O), sulfur (32S, 34S), and nitrogen (14N, 15N).

The elements listed above are important constituents in biological systems and are also

involved in many geochemical reactions. Small differences in the concentration of isotopes

exist in chemically identical compounds because of differences in the origin or certain

processes that have occurred after the compounds was produced. These characteristics make

isotope analyses very useful for determining the source of certain compounds in the environment

and/or determining the geochemical reactions that have affected the concentration of the

compounds or materials of interest. The radioactive isotopes are often used to determine the

age of different types of materials.

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In most cases, elements like to have an equal number of protons and neutrons because this makes

them the most stable. Stable atoms have a binding energy that is strong enough to hold the protons

and neutrons together. Even if an atom has an additional neutron or two it may remain stable.

However, an additional neutron or two may upset the binding energy and cause the atom to become

unstable. In an unstable atom, the nucleus changes by giving off a neutron to get back to a balanced

state. As the unstable nucleus changes, it gives off radiation and is said to be radioactive.

Radioactive isotopes are often called radioisotopes.