Chapter 32 - Modern Physics

• Rutherford Model of the Atom• Bohr Model of Atom

– Quantized Energy Levels

– Atomic Spectra

• Quantized Energy levels and Electron Waves• Quantum Mechanics

• Discovery of the Electron– A cathode ray is a beam of electrons that moves between metal plates

in an evacuated tube from a negative to a positive terminal.

• The electron beam is seen as a green beam.

– These rays can be deflected by a magnet.

• In 1897 JJ Thompson placed a positively charged plate on one side of the tube and a negatively charged plate on the other side of the tube.

• The beam was deflected away from the negative plate toward the positive plate.

• Thompson realized that the particles that made up the beam must be negatively charged, since like charges repel and opposite charges attract.

• By balancing the deflections made by the magnet with that made by the electrical field, Thompson was able to calculate the ratio of the charge to mass of an electron as 1.7584 X 1011 coulomb/kilogram

• These particles were later named electrons.

Discovery of The Electron

Charge of an Electron

1911 E. Rutherford

• Gold thin foil experiment with alpha radiation through gold foil proved that the atom consisted with a very dense nucleus. Based on the particles deflection pattern, Rutherford calculated the fraction of the atomic volume occupied by the positive nuclei, that is 1 part in 1014. The mass is so dense that a nucleus the size of a pea would have mass greater that 250 million tons. this led to the modern view of the atom.

1911 E. Rutherford

Atomic Spectra

Bohr Model of the Atom

Electron “Death” and “Resurrection”

Atomic Excitation and Decay

Bohr Atom Explained

= h/p

Electron Orbits

Atomic Spectra Explained

Quantum Mechanics

Correspondence Principle

• Special Relativity => Newtonian Mechanics if v << c• General Relativity => Universal Gravitation if m < huge!• Quantum Mechanics => Newtonian Mechanics as d, m increase• etc

Clicker Questions

1) In the Bohr model of hydrogen, discrete radii and energy states result when an electron circles the atom in an integral number of

A) diffraction patterns. B) wave frequencies.C) de Broglie wavelengths. D) none of these

2) Some alpha particles fired through a gold foil bounce backward by A) electrostatic repulsion with the electron clouds of gold atoms.B) electrostatic repulsion when close to gold nuclei.C) making direct hits with gold atoms.D) all of theseE) none of these

Clicker Questions

3) According to Niels Bohr, an electron in an excited state could give off

A) several photons in a series of transitions to the ground state.B) a continuous cascade of photons for a high-level transition.C) at most a single photon until the atom was excited again.D) none of these

4) According to the correspondence principle, a new theory is valid if it A) accounts for confirmed results from the old theory.B) predicts the same correct results as the old theory.C) overlaps and agrees where the old theory works.D) all of theseE) none of these