Phys141 Principles of Physical Science Chapter 9 Atomic Physics

1. Phys141 Principles of Physical ScienceChapter 9 Atomic Physics Instructor: Li Ma Office: NBC 126 Phone: (713) 313-7028 Email: malx@tsu.edu Webpage: http://itscience.tsu.edu/ma Department of Computer Science & Physics Texas Southern University, Houston Nov. 1, 2004

2. Topics To Be Discussed • Early Concepts of the Atom • The Dual Nature of Light • Bohr Theory of the Hydrogen Atom (only emphasize on some parts) • Skip §9.4 to §9.7

3. Development of Physics • Classical physics or Newtonian physics • Prior to about 1900 • Description and explanation of large-scale, observable phenomena – Macrocosm • Modern physics • Since about 1900 • Description and explanation of submicroscopic phenomena – Microcosm

4. Early Concepts of the Atom • About 400 B.C., debate on whether matter was continuous or discrete • Pluto & Aristotle: continuous • In 1807, John Dalton presented evidence that matter is discrete – “billiard ball model” • Each chemical element is composed of small, indivisible particles called atoms, which are identical for that element but different from atoms for other elements • Featureless, indivisible spheres of uniform density

5. Early Concepts of the Atom (cont) • In 1903, J.J. Thompson discovered the electron – “plum pudding model” • Atoms of all types contain electrons • An electron has mass of 9.11 x 10-31 kg and a charge of -1.6 x 10-19 C • An atom was much like a sphere of plum pudding, in which the electrons were just like raisins stuck randomly • In 1911, Ernest Rutherford – “nuclear model” • Tiny, but heavy, positively charged core and circulating electrons

6. The Dual Nature of Light • A radical development about nature of light • According to the classical theory, light is considered as a wave with a continuous flow of energy – wave nature • But there were some dilemma • Then light has another nature so the energy is discrete – particle nature

7. The Dual Nature of Light (cont) • Before the turn of 20th century • Visible light of all frequencies was emitted by the atoms of an incandescent solid • As the temperature increases, more radiation is emitted, and the component of maximum intensity is shifted to a higher frequency • Ultraviolet catastrophe • Discrepancy: the energy intensity that was actually observed was very much less than theory predicted

8. The Dual Nature of Light (cont) • Classically, an electron oscillator may vibrate at any frequency or have any energy up to some maximum value, but it can not explain that phenomena • Max Planck (1900) introduced a radical idea to explain the observed distribution of thermal radiation intensity • First step, Planck set toward a new theory called quantum physics

9. The Dual Nature of Light (cont) • Planck’s hypothesis: • Energy is quantized, i.e. an oscillator can only have discrete amounts of energy which depends on its frequency: Energy = Planck’s constant x frequency E = h·f where h = 6.63 x 10-34J-s • Planck introduced the idea of a quantum – a discrete amount of energy

10. The Dual Nature of Light (cont) • In the latter part of the 19th century • Electrons are emitted when certain metals are exposed to light • Photoelectric effect: • Direct conversion from light (radiant energy) to electrical energy • Only light above a certain frequency would cause electrons to be emitted

11. The Dual Nature of Light (cont) • Einstein solved this problem by introducing photon – a quantum of electromagnetic radiation (in 1905) • applying Planck’s hypothesis, Einstein decided that light is quantized and consists of “particles” of energy, rather than wave • The higher the frequency of the light, the greater the energy of its photons • By considering light to be composed of photons, Einstein was able to explain photoelectric effect

12. The Dual Nature of Light (cont) • Dual nature of light: to explain various phenomena, light must be described sometimes as a wave and sometimes as a particle • Use light’s wave nature to explain wave effects like diffraction and interference • Use light’s particle nature to explain ultraviolet catastrophe & photoelectric effect

13. Bohr Theory of the Hydrogen Atom • Line emission spectrum & line absorption spectrum • Discrete spectrum: 4 discrete wavelengths of visible light • Only certain wavelengths of visible light radiate energy

14. Bohr Theory of the Hydrogen Atom (cont) • Bohr’s atom model – planetary model • For hydrogen atom, the nucleus is a single proton, its one electron revolves around the nuclear proton in a circular orbit • The hydrogen electron could be only in discrete (specific) orbits with particular radii

15. Bohr Theory of the Hydrogen Atom (cont) • Bohr’s theory to explain the line spectra (discrete line spectrum) • The hydrogen electron does not radiate energy when in an allowed, discrete orbit but does so only when it makes a quantum jump, or transition, form one allowed orbit to another • A particular jump corresponds to a spectral line in line spectrum

16. Assignment • Homework Assignment • Review Questions (page 222): • 1,3,5,7,9,11,15 • Exercises (page 224): • 2,4 • It’s due Monday, 11/15/04 • Reading Assignment • Chapter 10: §10.1 & §10.2