1. James T. Shipman Jerry D. Wilson Charles A. Higgins, Jr. Atomic Physics Chapter 9

2. Atomic Physics Classical physics (Newtonian physics) – development of physics prior to around 1900 Classical physics was generally concerned with macrocosm – the description & explanation of large- scale phenomena –cannon balls, planets, wave motion, sound, optics, electromagnetism Modern physics (after about 1900) – concerned with the microscopic world – microcosm – the subatomic world is difficult to describe with classical physics This chapter deals with only a part of modern physics called Atomic Physics – dealing with electrons in the atom.

3. Sec 9.1Early Concepts of the Atom Atom: Atomos (Gr) Dalton’s model: Atoms are like tiny ball bearings Section 9.1

4. Sec 9.1Early Concepts of the Atom J. J. Thompson discovered the electron Experiments by R. A. Milliken showed the electron had charge –The charge on the electron is 1.6x10 -19 Coulombs Thompson’s “Plum- pudding model”

5. Sec 9.1Early Concepts of the Atom Rutherford’s experiment (ch 10) showed that the atom was mostly empty space –Electrons circulate around a positively charged core

6. Sec 9.1Early Concepts of the Atom This is how the models evolved in the early years Section 9.1

7. Sec 9.2 The Dual Nature of Light- Classical Wave Theory of Light The key to further understanding of the atom is to study how it interacts with light –What is a blackbody? –My definition: Anything that is warm that glows A warm burner on an oven A fire The Sun A star The filament of a light bulb (incandescent) A HUMAN BEING! Section 9.2

8. Sec 9.2 The Dual Nature of Light- Classical Wave Theory of Light Red Hot Steel The radiation component of maximum intensity determines a hot solid’s color. Section 9.2

9. Sec 9.2 The Dual Nature of Light- Classical Wave Theory of Light A blackbody’s temperature determines the wavelength of it’s brightest color –When heated, a blackbody glows brighter and more blue –When cooled, a blackbody glows dimmer and more red Section 9.2

10. Sec 9.2 The Dual Nature of Light- Classical Wave Theory of Light At shorter wavelengths (blue), classical wave theory says blackbodies should be bright. This is NOT what is actually observed. Section 9.2

11. Sec 9.2 The Dual Nature of Light- Classical Wave Theory of Light Max Planck’s model of a blackbody: He proposed that black bodies should be composed of a very large number of single cavity resonators. –Each resonator was like the standing waves on a vibrating string His radical assumptions: –Energy was transferred between blackbody and wave essentially instantaneously –Energy is transformed not continuously, but in discreet amounts (packets) he called quanta –The energy contained in each quanta was proportional to the frequency of the wave: E = hf

12. Sec 9.2 The Dual Nature of Light- Classical Wave Theory of Light Planck’s constant, h = 6.63x10 -23 Joule-sec This basically said that energy does not transform from one object to another, or from one form to another continuously, as one pours water from one cup to the another, but in discreet amounts that depended on the frequency of the wave. –We say that energy is thus “quantized”. Section 9.2

13. Sec 9.2 The Dual Nature of Light- Classical Wave Theory of Light Continuous Energy Section 9.2 Quantized Energy

14. Sec 9.2 The Dual Nature of Light- Classical Wave Theory of Light Another discovery was the photo-electric effect Certain metals emit electrons when exposed to light– The photoelectric effect This direct conversion of light into electrical energy is the basis for photocells –Automatic door openers, light meters, solar energy applications Once again classical wave theory could not explain the photoelectric effect Section 9.2

15. Sec 9.2 The Dual Nature of Light- Classical Wave Theory of Light Einstein took Planck’s concept of quanta and quantization one step further –The metal converted energy from the wave into energy of free electrons by the absorption of “packets” –Einstein called these packets of energy “photons” –Higher frequency, higher the energy absorbed. So what is light? Is it a wave or a particle? It is both Section 9.2

16. Sec 9.2 The Dual Nature of Light- Classical Wave Theory of Light

17. Section 9.3 Bohr Theory of the Hydrogen Atom Recall from chapter 7 that white light can be dispersed into a spectrum of colors by a prism –Due to differences in refraction for the specific wavelengths –This is a continuous spectrum

18. Section 9.3 Bohr Theory of the Hydrogen Atom In the late 1800’s experimental work with gas-discharge tubes revealed two other types of spectra –Line emission spectra displayed only bright spectral lines of certain frequencies –Line absorption spectra displays dark lines of missing colors

19. Section 9.3 Bohr Theory of the Hydrogen Atom In the late 1800’s experimental work with gas-discharge tubes revealed two other types of spectra –Line emission spectra displayed only bright spectral lines of certain frequencies –Line absorption spectra displays dark lines of missing colors

20. Sec 9.3 Bohr Theory of the Hydrogen Atom In 1913 an explanation of the observed spectral line phenomena was advanced by the Danish physicist Niels Bohr Section 9.3

21. Sec 9.3 Bohr Theory of the Hydrogen Atom Bohr predicted that the single hydrogen electron would only be found in discrete orbits with particular radii –Bohr’s possible electron orbits were given whole-number designations, n = 1, 2, 3, … Section 9.3

22. Sec 9.3 Bohr Theory of the Hydrogen Atom Bohr predicted that the single hydrogen electron would only be found in discrete orbits with particular radii –“n” is called the principal quantum number –The lowest n-value, (n = 1) has the smallest radius Section 9.3

23. Sec 9.3 Bohr Theory of the Hydrogen Atom In the Bohr model, each allowed orbit has a certain energy. –n = 1 is the lowest energy: It is the ground state – n = 2, 3, 4, … are the ‘excited states’. –All orbits are trapped in a ‘well’.

24. Sec 9.3 Bohr Theory of the Hydrogen Atom A transition to a lower energy level results in the emission of a photon. A transition to a higher energy level results in the absorption of a photon. Section 9.3

25. Sec 9.3 Bohr Theory of the Hydrogen Atom Transitions among discrete energy orbit levels give rise to discrete spectral lines within the UV, visible, and IR wavelengths Section 9.3

26. Sec 9.6 Matter Waves - de Broglie Waves With the development of the dual nature of light it became apparent that light “waves” sometime act like particles. Could the reverse be true? Can particles have wave properties? In 1925 the French physicist de Broglie postulated that matter has properties of both waves and particles. This is de Broglie’s hypothesis. Section 9.6 Skip section 9.4, and 9.5

27. Sec 9.6 Matter Waves - de Broglie Waves De Broglie’s hypothesis: Any moving particle has a wave associated with it whose wavelength is given by the following formula = h/mv = wavelength of the moving particle m = mass of the moving particle v = speed of the moving particle h = Planck’s constant (6.63 x 10 -34 J. s) Section 9.6

28. Sec 9.6 Matter Waves - de Broglie Waves In 1927 two U.S. scientists, Davisson and Germer, experimentally verified that particles have wave characteristics. These two scientists showed that a bean of electrons (particles) exhibits a diffraction pattern (a wave property.) Recall Section 7.4 – appreciable diffraction only occurs when a wave passes through a slit of approximately the same width as the wavelength Section 9.6

29. Sec 9.6 Matter Waves - de Broglie Waves X-Ray pattern Section 9.6 Diffraction pattern of electrons

30. Sec 9.7 Electron Cloud Model of an Atom Recall that Bohr chose to analyze the hydrogen atom, because it is the simplest atom It is increasingly difficult to analyze atoms with more that one electron, due to the myriad of possible electrical interactions In large atoms, the electrons in the outer orbits are also partially shielded from the attractive forces of the nucleus Section 9.7

31. Sec 9.7 Electron Cloud Model of an Atom With the discovery of the dual natures of both waves and particles … A new kind of physics was developed, called quantum mechanics or wave mechanics –Developed in the 1920’s and 1930’s as a synthesis of wave and quantum ideas Quantum mechanisms also integrated Heisenberg’s uncertainty principle –The concept of probability replaced the views of classical mechanics in describing electron movement Section 9.7

32. Sec 9.7 Electron Cloud Model of an Atom In 1926, the Austrian physicist Erwin Schrödinger presented a new mathematical equation applying de Broglie’s matter waves. Schrödinger’s equation was basically a formulation of the conservation of energy The simplified form of this equation is … ( E k + E p )  = E  – E k, E p, and E are kinetic, potential, and total energies, respectively –  = wave function Section 9.7

33. Sec 9.7 Electron Cloud Model of an Atom For the electron wave to be stable, the circumference of the orbit must be a whole number of wavelengths Section 9.7