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This simulation (the original PhET Java version) promotes understanding of isotopes by providing a simple way to model isotopes of the first 10 elements in the Periodic Table. In the most basic model, users click on an atomic symbol. The simulation displays a stable isotope for that atom. (For example, choose Helium and view a nucleus with two protons and two neutrons.) Now, drag neutrons into the nucleus and watch to see if atom becomes unstable. Students may be surprised to see that Beryllium and Flourine, for example, are unstable with equal numbers of protons and neutrons in the nucleus. Click on "Abundance in Nature" to see how common or rare a particular isotope is in nature. Mass number and Atomic Mass (amu) are displayed in real time.

See Related Materials for a lesson plan developed specifically for use with the "Isotopes and Atomic Mass" simulation.

This item is part of a larger collection of simulations developed by the Physics Education Technology project (PhET).

Please note that this resource requires Java Applet Plug-in.
Subjects Levels Resource Types
Education Practices
- Active Learning
= Modeling
Modern Physics
- Nuclear Physics
= Models of the Nucleus
Other Sciences
- Chemistry
- High School
- Middle School
- Lower Undergraduate
- Instructional Material
= Activity
= Interactive Simulation
= Model
Intended Users Formats Ratings
- Learners
- Educators
- application/java
- text/html
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Free access
© 2011 University of Colorado at Boulder
Additional information is available.
atom, atom simulation, atomic mass, elements, isotope, nuclear properties, radioactivity, stable element, unstable element
Record Cloner:
Metadata instance created July 18, 2011 by Caroline Hall
Record Updated:
December 27, 2018 by Caroline Hall
Last Update
when Cataloged:
July 8, 2011
Other Collections:

AAAS Benchmark Alignments (2008 Version)

4. The Physical Setting

4D. The Structure of Matter
  • 6-8: 4D/M1a. All matter is made up of atoms, which are far too small to see directly through a microscope.
  • 6-8: 4D/M1b. The atoms of any element are like other atoms of the same element, but are different from the atoms of other elements.
  • 9-12: 4D/H1. Atoms are made of a positively charged nucleus surrounded by negatively charged electrons. The nucleus is a tiny fraction of the volume of an atom but makes up almost all of its mass. The nucleus is composed of protons and neutrons which have roughly the same mass but differ in that protons are positively charged while neutrons have no electric charge.
  • 9-12: 4D/H2. The number of protons in the nucleus determines what an atom's electron configuration can be and so defines the element. An atom's electron configuration, particularly the outermost electrons, determines how the atom can interact with other atoms. Atoms form bonds to other atoms by transferring or sharing electrons.
  • 9-12: 4D/H3. Although neutrons have little effect on how an atom interacts with other atoms, the number of neutrons does affect the mass and stability of the nucleus. Isotopes of the same element have the same number of protons (and therefore of electrons) but differ in the number of neutrons.
  • 9-12: 4D/H4. The nucleus of radioactive isotopes is unstable and spontaneously decays, emitting particles and/or wavelike radiation. It cannot be predicted exactly when, if ever, an unstable nucleus will decay, but a large group of identical nuclei decay at a predictable rate. This predictability of decay rate allows radioactivity to be used for estimating the age of materials that contain radioactive substances.
  • 9-12: 4D/H5. Scientists continue to investigate atoms and have discovered even smaller constituents of which neutrons and protons are made.

11. Common Themes

11B. Models
  • 6-8: 11B/M1. Models are often used to think about processes that happen too slowly, too quickly, or on too small a scale to observe directly. They are also used for processes that are too vast, too complex, or too dangerous to study.
  • 6-8: 11B/M4. Simulations are often useful in modeling events and processes.
11D. Scale
  • 6-8: 11D/M3. Natural phenomena often involve sizes, durations, and speeds that are extremely small or extremely large. These phenomena may be difficult to appreciate because they involve magnitudes far outside human experience.
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(PhET, Boulder, 2011), WWW Document, (
PhET Simulation: Isotopes and Atomic Mass-Java Version (PhET, Boulder, 2011), <>.
APA Format
PhET Simulation: Isotopes and Atomic Mass-Java Version. (2011, July 8). Retrieved April 18, 2024, from PhET:
Chicago Format
PhET. PhET Simulation: Isotopes and Atomic Mass-Java Version. Boulder: PhET, July 8, 2011. (accessed 18 April 2024).
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PhET Simulation: Isotopes and Atomic Mass-Java Version. Boulder: PhET, 2011. 8 July 2011. 18 Apr. 2024 <>.
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@misc{ Title = {PhET Simulation: Isotopes and Atomic Mass-Java Version}, Publisher = {PhET}, Volume = {2024}, Number = {18 April 2024}, Month = {July 8, 2011}, Year = {2011} }
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%T PhET Simulation: Isotopes and Atomic Mass-Java Version %D July 8, 2011 %I PhET %C Boulder %U %O application/java

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%0 Electronic Source %D July 8, 2011 %T PhET Simulation: Isotopes and Atomic Mass-Java Version %I PhET %V 2024 %N 18 April 2024 %8 July 8, 2011 %9 application/java %U

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PhET Simulation: Isotopes and Atomic Mass-Java Version:

Accompanies PhET Teacher Activities: Modeling Isotopes

This is a lesson plan appropriate for Grades 7-10, created specifically to accompany the "Isotopes and Atomic Mass" simulation. It is very effective at guiding beginning students through an exploration of the atomic nucleus and factors that affect the stability of an atom.

relation by Caroline Hall
Is By The Same Author and Covers a Similar Topic As PhET Simulation: Nuclear Fission

A link to the PhET simulation "Nuclear Fission", which would be beneficial as a companion resource to "Isotopes and Atomic Mass".

relation by Caroline Hall
Is Replaced By PhET Simulation: Isotopes and Atomic Mass-HTML Version

Link to the newer HTML5 version of this PhET simulation

relation by Caroline Hall

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