Scope, Sequence, and Coordination
A Framework for High School Science Education
Based on the National Science Education Standards
Evidence Revealing the Composition of the Atom
The Nuclear Atom and its Components: Electrons, Protons, and Neutrons
Convincing evidence exists today that matter is composed of minute particles called atoms. Experiments can be done with cathode-ray tubes, electroscopes, and radioactive isotopes to show that matter has small positively and negatively charged components. Each of these components has a measurable amount of mass and, except for the neutron, electrical charge.
More detailed experimental analyses by Thomson, Rutherford, Millikan, Bohr, Sommerfeld, Pauli, Hund, and many others have led to the creation of an atomic model consisting of a small positively charged nucleus surrounded by negatively charged electrons. The electrons occupy most of the space in the atom and are held to the nucleus by electrical forces of attraction.
Each electron in an atom has its own distinct amount of energy. In flames, electrons in atoms can gain enough energy to make transitions to higher energy levels. When they move back to their original levels, light is emitted having specific energies (corresponding to the specific wavelengths of light observed), in many cases giving an intense color to the flame.
Such observations led to a suggested model of the atom in which electrons have discrete amounts of energy. Quantitative aspects of this model work well for atoms with single outer electrons. However, observations of spectra of atoms with multiple outer electrons, and consideration of the wave properties of electrons, led to development of a wave-mechanical model for the electrons in atoms, with each electron in a stationary wave pattern called an orbital.
Atom, electrical charge, spectrum, electroscope, proton, electron, nucleus
Electron, energy level, orbital, radioactive isotopes, nucleus
Particle, wavelength uncertainty, quanta
Emission line spectra for elements, Hund=s rule
Atomic theory, wave-mechanical model, Thomson model, Rutherford model, Bohr model, Sommerfeld model, Pauli exclusion principle, Heisenberg uncertainty principle, Planck=s theory, de Broglie relation