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PH 103

PH 103. Dr. Cecilia Vogel Lecture 21. Review. Spectra of hydrogen of multi-electron atoms. Outline. Fluorescence Nuclei properties composition, N, Z, A energy. photon. e -. e -. Transition Up. Electron absorbs energy perhaps from a photon goes to a higher energy level.

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PH 103

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  1. PH 103 Dr. Cecilia Vogel Lecture 21

  2. Review • Spectra • of hydrogen • of multi-electron atoms Outline • Fluorescence • Nuclei • properties • composition, N, Z, A • energy

  3. photon e- e- Transition Up • Electron absorbs energy • perhaps from a photon • goes to a higher energy level

  4. photon e- e- Transition Down • Electron loses energy to a photon -- gives off light • Electron goes to a lower energy level • losing energy

  5. Fluorescence • Fluorescent material excited by • absorbing lightof short wavelength, high frequency, like UV • Fluorescent material then de-excites by • emitting longer wavelength light • twice • Two photons are emitted • total energy of the two photons = energy of the absorbed UV photon

  6. visible photon visible photon UV photon e- Fluorescence • Fluorescent material excited by absorbing UV photon, then de-excites by emitting two visible photons • demo

  7. Fluorescent lights • Mercury discharge tube is basis • Hg excited by electric discharge • Hg gives off many wavelengths • including UV • The UV given off by Hg • excites the fluorescent material on the tube’s surface • which in turn gives off visible light (fluoresces) • that’s what we see

  8. Nucleus • Very small • size is several Fermi • = femtometer = fm = 10-15 m • compare to size of atom = several Å = 10-10 m • If the atom is scaled up to the size of the Earth, the nucleus would scale up to the size of a house. • Very heavy • About 99.98% of the mass of the atom is nucleus. • Nucleus about 4000 times as massive as electrons.

  9. Nucleus • Very dense • If this building were as dense as nucleus, it would have as much mass as the whole Earth! • Positively charged • the charge of a nucleus is +Ze • Z= atomic number of element • so with Z electrons (-Ze) • the atom is neutral • Made up of protons and neutrons • together protons and neutrons are called nucleons

  10. Protons and Neutrons • Proton • positive charge +e • Neutron • zero charge, neutral • Both proton and neutron • have mass almost 2000 times the electron’s mass

  11. Counting Protons • How many protons in nucleus • charge of nucleus is +Ze, so • The number of protons must be • =Z • = atomic number • depends only on element • ex – all carbon atoms have 6 protons • no matter what isotope • no matter what ion

  12. Counting Neutrons • How many neutrons in nucleus • many possibilities for each element. • Different isotopes of same element have different numbers of neutrons • N = neutron number • ex: A nucleus with 6 protons and 6 neutrons is different from a nucleus with 6 protons and 7 neutrons • They are different isotopes of carbon • differ in # of neutrons

  13. Counting Nucleons • How many nucleons in the nucleus • Let A = Z + N. • A = “mass number” • This is not the mass of the nucleus!!! • This is NOT the mass of the nucleus!!!

  14. Notation • C is atomic symbol for carbon • pre-subscript is the atomic number • 6 protons • subscript is the neutron number • 7 neutrons • pre-superscript is the mass number • 13 total nucleons • Don’t need to give N, can find it from • N = A - Z (N = 13 - 6 = 7) • Don’t need to give Z, can find it • in the periodic table (Carbon is Z=6)

  15. Strong Nuclear Force • Strong nuclear force is what holds the nucleus together, • “Nuclear” because it • acts between nucleons, • protons and neutrons alike; • electrons unaffected

  16. Strong Nuclear Force • Strong • must overcome electric repulsion, • and still hold the protons and neutrons in nucleus a million times stronger than electrons are held to atom. • Compare • a few eV to ionize an atom • several MeV to get a neutron or proton out of nucleus

  17. Energetics of Nucleus • The protons and neutrons in the nucleus have less energy than free protons and neutrons • it requires an input of energy to make them free • E(nucleus) < Z*E(free proton) + N* E(free neutron) • mnucleusc2 < Zmpc2 + Nmnc2 • mnucleus < Zmp + Nmn • mass of nucleus is less than the mass of its components!

  18. Binding Energy • The binding energy of atom • = energy it requires to break it apart into constituents • BE =[Z(mp+me)c2 + Nmnc2] – matomc2 • BE =[Z(m1H)c2 + Nmnc2] – matomc2

  19. Where to Find the Info • The only quantity of importance in nuclear physics that can be found in the periodic table is • the atomic number, Z • otherwise leave periodic table alone! • Most of the info we need is in Appendix B • Find the correct element • Within that element, find the correct isotope • Z and A both important • Can find mass in Appendix B • this is mass of atom with Z electrons

  20. Nuclear Units • Often use atomic mass units for mass (abbrev. amu or u) • Example mass of 107Ag is 106.905 u • What units do we get when we do mc2? • Example mass energy of 107Ag • = mc2= (106.905 u) c2. • = 106.905 uc2 • Do not leave these units in answers. • Convert: 1 uc2=931.5MeV • 106.905 uc2= 99582 MeV

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