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This model, rewritten to HTML5, 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. The newer HTML version has an additional model allowing students to create their own isotope mixtures to predict how average atomic mass changes by adjusting the isotope number.  

This item is part of a growing collection of simulations developed by the Physics Education Technology project (PhET), all available for free use. Registered users may also freely download lesson plans, video teaching tips, Power Point presentations, and other teacher-contributed material.

Please note that this resource requires Java Applet Plug-in.
Subjects Levels Resource Types
Education Practices
- Active Learning
= Modeling
Modern Physics
- Atomic Physics
= Atomic Models
- Nuclear Physics
= Models of the Nucleus
Other Sciences
- Chemistry
- High School
- Middle School
- Lower Undergraduate
- Instructional Material
= Activity
= Interactive Simulation
Appropriate Courses Categories Ratings
- Physical Science
- Physics First
- Conceptual Physics
- Algebra-based Physics
- AP Physics
- Activity
- New teachers
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Free access
© 2016 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 December 27, 2018 by Caroline Hall
Record Updated:
December 27, 2018 by Caroline Hall
Last Update
when Cataloged:
July 8, 2016

Next Generation Science Standards

Matter and Its Interactions (HS-PS1)

Students who demonstrate understanding can: (9-12)
  • Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms. (HS-PS1-1)

Disciplinary Core Ideas (K-12)

Structure and Properties of Matter (PS1.A)
  • Each atom has a charged substructure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons. (9-12)
Chemical Reactions (PS1.B)
  • The total number of each type of atom is conserved, and thus the mass does not change. (6-8)
  • The fact that atoms are conserved, together with knowledge of the chemical properties of the elements involved, can be used to describe and predict chemical reactions. (9-12)

Crosscutting Concepts (K-12)

Patterns (K-12)
  • Macroscopic patterns are related to the nature of microscopic and atomic-level structure. (6-8)
  • Different patterns may be observed at each of the scales at which a system is studied and can provide evidence for causality in explanations of phenomena. (9-12)
Scale, Proportion, and Quantity (3-12)
  • Phenomena that can be observed at one scale may not be observable at another scale. (6-8)
  • The significance of a phenomenon is dependent on the scale, proportion, and quantity at which it occurs. (9-12)
Structure and Function (K-12)
  • Complex and microscopic structures and systems can be visualized, modeled, and used to describe how their function depends on the shapes, composition, and relationships among its parts, therefore complex natural structures/systems can be analyzed to determine how they function. (6-8)
Scientific Knowledge Assumes an Order and Consistency in Natural Systems (1-12)
  • Science assumes that objects and events in natural systems occur in consistent patterns that are understandable through measurement and observation. (6-8)
  • Science assumes the universe is a vast single system in which basic laws are consistent. (9-12)

NGSS Science and Engineering Practices (K-12)

Developing and Using Models (K-12)
  • Modeling in 6–8 builds on K–5 and progresses to developing, using and revising models to describe, test, and predict more abstract phenomena and design systems. (6-8)
    • Develop a model to describe unobservable mechanisms. (6-8)
  • Modeling in 9–12 builds on K–8 and progresses to using, synthesizing, and developing models to predict and show relationships among variables between systems and their components in the natural and designed worlds. (9-12)
    • Develop a model based on evidence to illustrate the relationships between systems or between components of a system. (9-12)

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.
ComPADRE is beta testing Citation Styles!

Record Link
AIP Format
(PhET, Boulder, 2016), WWW Document, (https://phet.colorado.edu/en/simulation/isotopes-and-atomic-mass).
PhET Simulation: Isotopes and Atomic Mass-HTML Version (PhET, Boulder, 2016), <https://phet.colorado.edu/en/simulation/isotopes-and-atomic-mass>.
APA Format
PhET Simulation: Isotopes and Atomic Mass-HTML Version. (2016, July 8). Retrieved July 15, 2024, from PhET: https://phet.colorado.edu/en/simulation/isotopes-and-atomic-mass
Chicago Format
PhET. PhET Simulation: Isotopes and Atomic Mass-HTML Version. Boulder: PhET, July 8, 2016. https://phet.colorado.edu/en/simulation/isotopes-and-atomic-mass (accessed 15 July 2024).
MLA Format
PhET Simulation: Isotopes and Atomic Mass-HTML Version. Boulder: PhET, 2016. 8 July 2016. 15 July 2024 <https://phet.colorado.edu/en/simulation/isotopes-and-atomic-mass>.
BibTeX Export Format
@misc{ Title = {PhET Simulation: Isotopes and Atomic Mass-HTML Version}, Publisher = {PhET}, Volume = {2024}, Number = {15 July 2024}, Month = {July 8, 2016}, Year = {2016} }
Refer Export Format

%T PhET Simulation: Isotopes and Atomic Mass-HTML Version %D July 8, 2016 %I PhET %C Boulder %U https://phet.colorado.edu/en/simulation/isotopes-and-atomic-mass %O text/html

EndNote Export Format

%0 Electronic Source %D July 8, 2016 %T PhET Simulation: Isotopes and Atomic Mass-HTML Version %I PhET %V 2024 %N 15 July 2024 %8 July 8, 2016 %9 text/html %U https://phet.colorado.edu/en/simulation/isotopes-and-atomic-mass

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The AIP Style presented is based on information from the AIP Style Manual.

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PhET Simulation: Isotopes and Atomic Mass-HTML Version:

Replaces PhET Simulation: Isotopes and Atomic Mass-Java Version

Link to the older Java Version of this PhET simulation.

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