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published by the Physics Education Technology Project
written by Robert Marrero
This lesson plan was created specifically to accompany the PhET simulation "Isotopes and Atomic Mass". Appropriate for grades 7-10, it provides explicit guidance for modeling atoms and their variant isotopes. As students add neutrons to the nuclear model, they can see a relationship between the stability of the atom and its abundance in nature. The model makes it easy to visualize that atoms of one element always have the same number of protons, but can have various numbers of neutrons. It will help students differentiate Atomic Number (number of protons in the nucleus) from Mass Number (the number of protons and neutrons.

The isotope simulation, which must be open and displayed to complete this activity, is available from PhET at: Isotopes and Atomic Mass.

This lesson is part of PhET (Physics Education Technology Project), a large collection of free interactive simulations for science education.
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- Active Learning
= Modeling
Modern Physics
- Nuclear Physics
= Models of the Nucleus
Other Sciences
- Chemistry
- Middle School
- High School
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© 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:
August 18, 2016 by Lyle Barbato
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when Cataloged:
July 8, 2011
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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.

10. Historical Perspectives

10G. Splitting the Atom
  • 9-12: 10G/H5. Radioactivity has many uses other than generating energy, including in medicine, industry, and scientific research in many different fields.

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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AIP Format
R. Marrero, (Physics Education Technology Project, Boulder, 2011), WWW Document, (
R. Marrero, PhET Teacher Activities: Modeling Isotopes, (Physics Education Technology Project, Boulder, 2011), <>.
APA Format
Marrero, R. (2011, July 8). PhET Teacher Activities: Modeling Isotopes. Retrieved January 19, 2017, from Physics Education Technology Project:
Chicago Format
Marrero, Robert. PhET Teacher Activities: Modeling Isotopes. Boulder: Physics Education Technology Project, July 8, 2011. (accessed 19 January 2017).
MLA Format
Marrero, Robert. PhET Teacher Activities: Modeling Isotopes. Boulder: Physics Education Technology Project, 2011. 8 July 2011. 19 Jan. 2017 <>.
BibTeX Export Format
@misc{ Author = "Robert Marrero", Title = {PhET Teacher Activities: Modeling Isotopes}, Publisher = {Physics Education Technology Project}, Volume = {2017}, Number = {19 January 2017}, Month = {July 8, 2011}, Year = {2011} }
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%A Robert Marrero
%T PhET Teacher Activities: Modeling Isotopes
%D July 8, 2011
%I Physics Education Technology Project
%C Boulder
%O application/ms-word

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%0 Electronic Source
%A Marrero, Robert
%D July 8, 2011
%T PhET Teacher Activities: Modeling Isotopes
%I Physics Education Technology Project
%V 2017
%N 19 January 2017
%8 July 8, 2011
%9 application/ms-word

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PhET Teacher Activities: Modeling Isotopes:

Accompanies PhET Simulation: Isotopes and Atomic Mass

This is a link to the PhET simulation "Isotopes and Atomic Mass", which this lesson was specifically created to accompany.

relation by Caroline Hall
Same topic as PhET Teacher Activities: Nuclear Fission Simulation - Student Guide

A good follow-up exercise that explores what happens in a nuclear reaction, as students fire accelerated neutrons into a Uranium-235 nucleus. Created for use with the PhET simulation Nuclear Fission.

relation by Caroline Hall

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