1. Phys141 Principles of Physical Science Chapter 9 Atomic Physics Instructor: Li Ma Office: NBC 126 Phone: (713) 313-7028 Email: malx@tsu.edumalx@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 20 th 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 -34 J-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 19 th 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