Unit 9: The Atom

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.

1. What are the characteristics properties of atoms?
2. How do scientists support the theory of the particulate nature of matter when the particles are too small to be observed directly?
3. If most matter is composed of electrically charged particles, why does only some matter show evidence of charges?
4. What evidence leads scientists to describe subatomic particles as having both wave and particle characteristics?
5. What is the source of radioactivity and nuclear energy?