Notes 38 - Topic 7 - Atomic and Nuclear Physics

1. Notes 38 - Topic 7 - Atomic and Nuclear Physics ------------------------------------------------------------------------------------------------- The History of the Atomic Model http://library.thinkquest.org/C006669/data/Chem/atomic/development.html • ”The Raisin Pudding Model of the Atom” - In the final 19th Century non-experimental model, the atom was thought to be like a pudding with raisins, where the pudding was positive and the raisins were negative. • In 1897, J. J. Thomson discovered that “cathode rays” were really made of negatively charged particles...he called them “corpuscles.” It was George FitzGerald (Scotland) who actually applied the name “electron” to Thomson’s corpuscles. Thomson received the Nobel Prize in Physics in 1906 for his discovery. J. J. Thomson http://www.aip.org/history/electron/jjthomson.htm Thomson model

2. 7.1.1Nuclear Model of the Atom; The current model of the atom features a small, positively charged nucleus with protons and neutrons surrounded by negatively charged electrons. 7.1.2Supporting Evidence for the Nuclear Model; • Beginning in 1907, Ernest Rutherford (New Zealand) directed Hans Geiger and his grad student Ernest Marsden to conduct the famous “gold foil experiment:” A very thin sheet of gold foil ( 400 atoms thick) was bombarded by alpha particles (Helium nuclei = 2+) from a radioactive source. A zinc sulfide screen surrounded the foil, and each alpha particle that struck the screen generated a tiny flash of light. Most alpha particles went straight through...but some were deflected by as much as 180 degrees.

3. • Rutherford discovered the existence of a “nucleus” with all of the + charge and most of the mass of the atom. He received the 1908 Nobel Prize in chemistry for his discovery. He theorized the existence of the neutron with mass but no charge in the nucleus.

4. •The Planetary Model of the Atom- The atom was now thought to resemble a solar system with the protons of the nucleus in the center and the electrons circling like planets. • In 1909, Robert Millikan performed a classic experiment in which he measured the charge-to-mass ratio of the electron. He also found out that charge was “quantized,” ie., charge existed in whole number multiples of a single lowest charge. He was awarded the Nobel Prize for Physics in 1923 for his discovery. Millikan Experiment http://www.juliantrubin.com/bigten/millikanoildrop.html

7. 7.1.3 Limitations to the early Rutherford “Planetary” Model A. Maxwell demonstrated that accelerating electrons give off energy. How are atomic electrons able to keep their energy and not spiral quickly into the nucleus to convert protons into neutrons and “kill” the atom? B. How can the nucleus be stable if it contains positively charged protons that exert strong Coulomb forces of repulsion on each other? C. How does this model explain the existence of atoms with the same chemical properties but different atomic masses?

8. 7.1.4 Evidence for the Electron Energy Levels • In the mid-1700’s Thomas Melville (Scotland) discovered that different elements give off different spectra of light when heated, with each element having a unique “fingerprint” of light; Emission & Absorption Spectra http://jersey.uoregon.edu/vlab/elements/Elements.html

9. • The Bohr Model of the Atom - the energy of the electrons in the Bohr atom is restricted to certain discrete values; - energy is quantized; - only certain orbits with certain radii are allowed; - orbits in between simply don't exist; - therefore, only certain energies can be absorbed and emitted; - the unique spectrum of each atom which is produced by its electrons moving from lower energy levels to higher (absorption spectra) and higher to lower energy level (emission spectra). Bohr Model http://csep10.phys.utk.edu/astr162/lect/light/bohr.html • But...why do these energy levels for electrons exist?

11. • Einstein theorizes the photon - In 1905, Albert Einstein, based on the work of Max Planck, theorized that light is made of small packets of energy called photons which behave according to the equation... E = h f ...where E is energy in eV, f is the frequency of the light, and h is Planck’s Constant... h = 6.6 x 10-34 Js; - this perfectly fits the Bohr Model since every electron that jumps from the same energy level would have the same frequency (aka color); - photons produced by energy level jumps by atomic electrons exactly correspond in wavelength and frequency to those predicted by Einstein’s equation; • But...why do these energy levels for electrons exist?

12. • The Quantum Mechanics Model of the Atom - atomic energy levels are caused by standing matter waves of electrons; Louis de Broglie http://www.spaceandmotion.com/Physics-Louis-de-Broglie.htm