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

• 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.

• 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.

This teacher-created lesson plan accompanies the PhET simulation Isotopes and Atomic Mass. 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 from mass number. The explicit guidance makes it appropriate for middle school and conceptual physics courses.

• Physical Sciences K-8 Matter and Interactions Unit
• Physics First Matter and Interactions Unit
• Conceptual Physics Matter and Interactions Unit

This teacher-created lesson plan accompanies the PhET simulation Isotopes and Atomic Mass. 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 from mass number. The explicit guidance makes it appropriate for middle school and conceptual physics courses.

• Physical Sciences K-8 Molecular Structures and Bonding Unit
• Physics First Molecular Structures and Bonding Unit
• Conceptual Physics Molecular Structures and Bonding Unit

This teacher-created lesson plan accompanies the PhET simulation Isotopes and Atomic Mass. 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 from mass number. The explicit guidance makes it appropriate for middle school and conceptual physics courses.

• Physical Sciences K-8 Nuclear Processes Unit
• Physics First Nuclear Processes Unit
• Conceptual Physics Nuclear Processes Unit