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
Matter is made of minute particles called atoms, and atoms are composed of even smaller components. These components have measurable properties, such as mass and electrical charge. Each atom has a positively charged nucleus surrounded by negatively charged electrons. The electric force between the nucleus and electrons holds the atom together.

Further Description:

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.

Concepts Needed:

Grade 9

Atom, electrical charge, spectrum, electroscope, proton, electron, nucleus

Grade 10


Grade 11

Electron, energy level, orbital, radioactive isotopes, nucleus

Grade 12

Particle, wavelength uncertainty, quanta

Empirical Laws or Observed Relationships:

Emission line spectra for elements, Hund=s rule

Theories or Models:

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

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Micro-Unit Description:

Evidence Revealing the Composition of the Atom
Students should relate electrical properties of matter in a qualitative way to an atomic model. They should compare J. J. Thomson's model of the atom to that subsequently proposed by Rutherford and examine the evidence that led to the rejection of the Thomson model and acceptance of the Rutherford nuclear model; the atom consists of a nucleus containing most of the mass and with a positive charge, around which move enough electrons to make the atom electrically neutral.

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