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from A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (pages 111-112)
Phenomena involving nuclei are important to understand, as they explain the formation and abundance of the elements, radioactivity, the release of energy from the sun and other stars, and the generation of nuclear power. To explain and predict nuclear processes, two additional types of interactions—known as strong and weak nuclear interactions—must be introduced. They play a fundamental role in nuclei, although not at larger scales because their effects are very short range.
The strong nuclear interaction provides the primary force that holds nuclei together and determines nuclear binding energies. Without it, the electromagnetic forces between protons would make all nuclei other than hydrogen unstable. Nuclear processes mediated by these interactions include fusion, fission, and the radioactive decays of unstable nuclei. These processes involve changes in nuclear binding energies and masses (as described by E = mc2), and typically they release much more energy per atom involved than do chemical processes.
Nuclear fusion is a process in which a collision of two small nuclei eventually results in the formation of a single more massive nucleus with greater net binding energy and hence a release of energy. It occurs only under conditions of extremely high temperature and pressure. Nuclear fusion occurring in the cores of stars provides the energy released (as light) from those stars. The Big Bang produced matter in the form of hydrogen and smaller amounts of helium and lithium. Over time, stars (including supernova explosions) have produced and dispersed all the more massive atoms, starting from primordial low-mass elements, chiefly hydrogen.
Nuclear fission is a process in which a massive nucleus splits into two or more smaller nuclei, which fly apart at high energy. The produced nuclei are often not stable and undergo subsequent radioactive decays. A common fission fragment is an alpha particle, which is just another name for a helium nucleus, given before this type of “radiation” was identified.
In addition to alpha particles, other types of radioactive decays produce other forms of radiation, originally labeled as “beta” and “gamma” particles and now recognized as electrons or positrons, and photons (i.e., high-frequency electromagnetic radiation), respectively. Because of the high-energy release in nuclear transitions, the emitted radiation (whether it be alpha, beta, or gamma type) can ionize atoms and may thereby cause damage to biological tissue.
Nuclear fission and radioactive decays limit the set of stable isotopes of elements and the size of the largest stable nucleus. Spontaneous radioactive decays follow a characteristic exponential decay law, with a specific lifetime (time scale) for each such process; the lifetimes of different nuclear decay processes range from fractions of a second to thousands of years. Some unstable but long-lived isotopes are present in rocks and minerals. Knowledge of their nuclear lifetimes allows radiometric dating to be used to determine the ages of rocks and other materials from the isotope ratios present.
In fission, fusion, and beta decay processes, atoms change type, but the total number of protons plus neutrons is conserved. Beta processes involve an additional type of interaction (the weak interaction) that can change neutrons into protons or vice versa, along with the emission or absorption of electrons or positrons and of neutrinos. Isolated neutrons decay by this process.
from NGSS Appendix E: Disciplinary Core Idea Progressions
from A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (page 113)
By the end of grade 2. [Intentionally left blank.]
By the end of grade 5. [Intentionally left blank.]
By the end of grade 8. Nuclear fusion can result in the merging of two nuclei to form a larger one, along with the release of significantly more energy per atom than any chemical process. It occurs only under conditions of extremely high temperature and pressure. Nuclear fusion taking place in the cores of stars provides the energy released (as light) from those stars and produced all of the more massive atoms from primordial hydrogen. Thus the elements found on Earth and throughout the universe (other than hydrogen and most of helium, which are primordial) were formed in the stars or supernovas by fusion processes.
By the end of grade 12. Nuclear processes, including fusion, fission, and radioactive decays of unstable nuclei, involve changes in nuclear binding energies. The total number of neutrons plus protons does not change in any nuclear process. Strong and weak nuclear interactions determine nuclear stability and processes. Spontaneous radioactive decays follow a characteristic exponential decay law. Nuclear lifetimes allow radiometric dating to be used to determine the ages of rocks and other materials from the isotope ratios present.
Normal stars cease producing light after having converted all of the material in their cores to carbon or, for more massive stars, to iron. Elements more massive than iron are formed by fusion processes but only in the extreme conditions of supernova explosions, which explains why they are relatively rare.
A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (pages 111-113)