What characteristics of atoms do scientists use to understand matter?
In this unit students investigate the nature of the atom and how the particulate model of matter has developed. The fundamental purpose of this unit is to introduce students to this model and help them understand how this theory was developed and modified to account for everyday observations of the material world. Because atoms, the particles of the model, are extremely small and cannot be observed directly without sophisticated equipment, this unit will often use reading about the evidence collected by the scientists who were originally involved in making and interpreting these observations. Students can make certain observations and then use them to infer the validity and usefulness of the model. First, students explore the nature of electrical chargers and the nature of the forces between them. They then draw charge distribution diagrams of various charged and neutral objects. Then students are introduced to Coulomb’s Law and the electrical nature of all matter. Next they investigate the inferential methods used to measure the size of the atom and their electrical charge by using pennies hidden in film containers. The classic experiments of Rutherford and Millikan are introduced to account for the charge on the electron and the size of the nucleus. Students also observe with simple spectrometers the spectra emitted from H, He, and Ne sources. They review the model of the atom developed by Bohr and how it is used to explain ionization energy and atomic structure. Next the wave nature of electrons is introduced to account for the photoelectric effect. Then the further modifications of the basic Bohr model of the atom required by the wave-particle duality theory and the work of Schrodinger and others who developed quantum mechanics are introduced. Last the nucleus of the atom is introduced and how a very strong nuclear force is required to account for its small size, very large density with a strong electrostatic repulsive force. This then leads to investigating the probabilistic nature of radioactive decay using sugar cubes. A concluding discussion contrasting nuclear fusion and fission explores the mass-energy relationship in nuclear reactions and their potential as an energy source for a world that now relies on fossil fuels.
C4.8A Identify the location, relative mass, and charge for electrons, protons, and neutrons.
C4.8B Describe the atom as mostly empty space with an extremely small, dense nucleus consisting of the protons and neutrons and an electron cloud surrounding the nucleus.
C4.8C Recognize that protons repel each other and that a strong force needs to be present to keep the nucleus intact.
STANDARD P3: FORCES AND MOTION Students identify interactions between objects either as being by direct contact (e.g., pushes or pulls, friction) or at a distance (e.g., gravity, electromagnetism), and to use forces to describe interactions between objects. They recognize that non-zero net forces always cause changes in motion (Newton’s fi rst law). These changes can be changes in speed, direction, or both. Students use Newton’s second law to summarize relationships among and solve problems involving net forces, masses, and changes in motion (using standard metric units). They explain that whenever one object exerts a force on another, a force equal in magnitude and opposite in direction is exerted back on it (Newton’s third law).
P3.1 Basic Forces in Nature
P3.1A Identify the force(s) acting between objects in “direct contact” or at a distance.
P3.1b Explain why scientists can ignore the gravitational force when measuring the net force between two electrons.
P3.1c Provide examples that illustrate the importance of the electric force in everyday life.
P3.1d Identify the basic forces in everyday interactions.
P3.7A Predict how the electric force between charged objects varies when the distance between them and/or the magnitude of charges change.
P3.7B Explain why acquiring a large excess static charge (e.g., pulling off a wool cap, touching a Van de Graaff generator, combing) affects your hair.
P3.7x Electric Charges — Interactions
P3.7c Draw the redistribution of electric charges on a neutral object when a charged object is brought near.
P3.7f Determine the new electric force on charged objects after they touch and are then separated.
P4.12A Describe peaceful technological applications of nuclear fission and radioactive decay.
P4.12B Describe possible problems caused by exposure to prolonged radioactive decay.
P4.12C Explain how stars, including our Sun, produce huge amounts of energy (e.g., visible, infrared, ultraviolet light).
P4.12d Identify the source of energy in fission and fusion nuclear reactions.
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- What are the characteristics properties of atoms?
- How do scientists support the theory of the particulate nature of matter when the particles are too small to be observed directly?
- If most matter is composed of electrically charged particles, why does only some matter show evidence of charges?
- What evidence leads scientists to describe subatomic particles as having both wave and particle characteristics?
- What is the source of radioactivity and nuclear energy?
distribution of electric charge
electric charge (positive and negative)
inverse square law
mass to energy conversion E = mc2
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