What is an isotope?

	Vocabulary
	alpha particle atomic number atomic weight beta particle element isotope isotopic abundance mass number neutron nuclide nuclide symbol proton radioactivity

Before the discovery of radioactivity, all atoms of a particular element were assumed to be identical. Pure substances always had the same characteristic chemical and physical properties, no matter where the sample was taken from or how it was obtained. If the samples are indistinguishable, scientists reasoned, the atoms or molecules making up the samples should also be indistinguishable.

When evidence began to mount that an element's radioactive properties might vary from sample to sample, scientists were astonished. Thorium, for example, seemed to have at least two varieties. Thorium in naturally occurring minerals like thorite (ThSiO₄) spits out alpha particles (helium nuclei). But thorium isolated from decaying uranium emits beta particles (electrons). Otherwise, the properties of the two forms of thorium are identical. This was an unsettling discovery, because it was in direct conflict with one of the foundation postulates of Dalton's atomic theory.

If the two forms have identical chemical properties, their electronic structures must also be identical. If both forms have the same number of electrons, then both must have the same number of protons in their nuclei. But something had to be different about them, or they wouldn't emit different types of radiation.

Isotopes have the same
atomic number but
different mass numbers.
Experiments with mass spectrometry revealed that each of these element forms had its own distinctive mass. The element forms were called isotopes, from the Greek words for "same place", because all of these forms fit into the same place on the periodic table. After neutrons were discovered, it was realized that isotopes are substances that have same number of protons in their nuclei, but different numbers of neutrons. Isotopes have the same atomic number but different masses and mass numbers.

Isotopy isn't limited to radioactive elements. Lighter elements almost always occur naturally as mixtures of isotopes. For example, hydrogen can occur in three isotopes:

hydrogen-1 nucleus Hydrogen-1 (sometimes called protium) has one proton and no neutrons in its nucleus. (Notice that isotopes are named by separating the element name and the mass number with a dash).

deuterium nucleus Hydrogen-2 (also called deuterium) has one proton and one neutron in its nucleus. In a natural sample of hydrogen one in every 6000 H atoms is hydrogen-2.

tritium nucleus Hydrogen-3 (also called tritium) is a radioactive isotope with one proton and two neutrons per nucleus. Tritium is present in extremely low concentrations in natural hydrogen, but can be produced in large quantities by nuclear weapons.

The atomic weight given for an element is actually an average of the isotope masses, not the weight of an individual atom of the element. For example, 98.89% of all carbon atoms are carbon-12 (with 6 protons and 6 neutrons, and a mass of 12.00000). Almost all of the remainder are carbon-13 (with 6 protons and 7 neutrons, and a mass of 13.003354). The atomic weight is 12.01, which is 0.9889×12.00000 + 0.0111×13.003354.

Resources

A History of Mass Spectrometry (Scripps Center for Mass Spectrometry)

Includes a timeline showing the evolution of mass spectrometry from J. J. Thomson's pioneering cathode ray studies to mass spectrometry of oligonuclides and viruses in the 1990's. The timeline provides abstracts and references for seminal papers.
http://masspec.scripps.edu/hist.html (06/18/01)

Elements, Isotopes, and Radioactivity (US Geological Survey)
Bruce Doe explains how isotopic analysis is used in finding the age of the earth, determining how recently earthquake faults have been active, and how often volcanos erupt and landslides occur. A variant of the mass spectrometer (the magnetic sector mass spectrometer)is used to collect the data.
http://minerals.cr.usgs.gov/gips/na/0radio.htm#radio

Exact Compound Masses (NCSU Mass Spectrometry Facility)
Computing molecular weights by adding up all the atomic weights given on a periodic table gives you the average molecular weight. Just as no chlorine atom has a mass of 35.453 amu, no molecule will have a mass equal to its average molecular weight. To get the exact mass of the most common isotopic variation of a compound, you have to add up the masses of only the most common isotopes. This page outlines the procedure for exact molecular mass calculations, and gives a table of most common isotopic masses determined by mass spectrometry.
http://ch9000.chem.ncsu.edu/Exact_Mass.html

Introduction to Mass Spectrometry (Scripps Center for Mass Spectrometry)
Briefly outlines the history and general principles of mass spectrometry.
http://masspec.scripps.edu/FINALCOPY.htm (06/18/01)

Isotope pattern calculator (Mark Winter, Sheffield University)
Type the formula of a chemical compound and get the isotope pattern of the parent peak on its mass spectrum. Part of Mark Winter's Chemputer project.
http://www.shef.ac.uk/chemistry/chemputer/isotopes.html (10/03/98, 10/23/99)

Isotopes: Better Migration Through Chemistry (The Why Files)
How isotopic analysis is being used to track migrating insects and birds to uncover reasons for declining populations.
http://whyfiles.news.wisc.edu/083isotope/index.html (3/29/99)

Martian Fossils (Dr. Jamie Love)
An article explaining how isotopic analysis was used to support the controversial evidence for fossilized life in a meteorite. Part of Dr. Love's Science Explained site.
http://www.synapse.ndo.co.uk/science/martian/index.html (10/11/98)

Table of Isotopes (Lawrence Berkeley National Laboratory)
An vast database of nuclear structure, abundance, and decay data. Data can be retrieved using the usual periodic table interface, or by a downloadable helper application called "Isotope Explorer"; by Java applets, or by search forms, or by downloading Adobe PDF files.
http://ie.lbl.gov/toi.htm (11/24/99)

The Nobel Prize in Chemistry 1921 (The Nobel Foundation)
Frederick Soddy was awarded the 1921 Nobel Prize in Chemistry "for his contributions to our knowledge of the chemistry of radioactive substances, and his investigations into the origin and nature of isotopes." Includes the full text of prize presentation speech and biographical information.
http://www.nobel.se/chemistry/laureates/1921/index.html (11/29/99)

The Nobel Prize in Chemistry 1922 (The Nobel Foundation)
William Francis Aston was awarded the 1922 Nobel Prize in Chemistry "for his discovery, by means of his mass spectrograph, of isotopes, in a large number of non-radioactive elements, and for his enunciation of the whole-number rule." Includes the full text of the prize presentation speech and biographical information.
http://www.nobel.se/chemistry/laureates/1922/index.html (11/29/99)

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! What is an isotope?