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Forms of the same element that contain different numbers of neutrons.

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Players in a football team have different weights and heights, like isotopes

Credit: Courtesy of Chief Photographer's Mate Johnny Bivera, U.S. Navy
Source: http://commons.wikimedia.org/wiki/File:Football_play_from_scrimmage.jpg
License: CC BY-NC 3.0

Are all the members of the football team shown above identical? 

They are on the same team and are all known by the same team name, but there are individual differences among the players.  We do not expect the kicker to be as big as the quarterback.  The tight end is very likely to weigh less than the defensive tackle on the other side of the ball.  They play as a unit, but they have different weights and heights.


The history of the atom is full of some of these differences.  Although John Dalton stated in his atomic theory of 1804 that all atoms of an element are identical, the discovery of the neutron began to show that this assumption was not correct. The study of radioactive materials  (elements that spontaneously give off particles to form new elements) by Frederick Soddy (1877-1956) gave important clues about the internal structure of atoms.  His work showed that some substances with different radioactive properties and different atomic weights were in fact the same element.  He coined the term isotope from the Greek roots isos (íσος “equal”) and topos (τóπος “place”). He described isotopes as, “Put colloquially, their atoms have identical outsides but different insides.” Soddy won the Nobel Prize in Chemistry in 1921 for his work.

As stated earlier, not all atoms of a given element are identical. Specifically, the number of neutrons can be variable for many elements. As an example, naturally occurring carbon exists in three forms. Each carbon atom has the same number of protons (6), which is its atomic number. Each carbon atom also contains six electrons in order to maintain electrical neutrality. However the number of neutrons varies as six, seven, or eight. Isotopes are atoms that have the same number atomic number, but different mass numbers due to a change in the number of neutrons.

The three isotopes of carbon can be referred to as carbon-12 (126C), carbon-13 (136C), and carbon-14 (146C) refers to the nucleus of a given isotope of an element. A carbon atom is one of three different nuclides. Most elements naturally consist of mixtures of isotopes. Carbon has three natural isotopes, while some heavier elements can have many more. Tin has ten stable isotopes, the most of any element.

While the presence of isotopes affects the mass of an atom, it does not affect its chemical reactivity. Chemical behavior is governed by the number of electrons and the number of protons. Carbon-13 behaves chemically in exactly the same way as the more plentiful carbon-12.


  • Isotopes are atoms that have the same atomic number, but different mass numbers due to a change in the number of neutrons.
  • The term nuclide refers to the nucleus of a given isotope of an element.
  • The atomic mass of an atom equals the sum of the protons and the neutrons.


  1. What does the term “isotope” mean
  2. Define “isotope.”
  3. What affects the chemical behavior of an atom?
  4. An isotope of yttrium has 39 protons and 59 neutrons. What is the atomic mass of that isotope?
  5. An isotope with an atomic mass of 193 has 116 neutrons. What is the atomic number of this isotope?
  6. An isotope of barium (atomic number 56) has an atomic mass of 138. How many neutrons are in the nucleus of this isotope?

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  1. [1]^ Credit: Courtesy of Chief Photographer's Mate Johnny Bivera, U.S. Navy; Source: http://commons.wikimedia.org/wiki/File:Football_play_from_scrimmage.jpg; License: CC BY-NC 3.0

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