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PH 103 Dr. Cecilia Vogel Lecture 21 Review Spectra of hydrogen of multi-electron atoms Outline Fluorescence Nuclei properties composition, N, Z, A energy Transition Up Electron absorbs energy perhaps from a photon goes to a higher energy level ee photon Transition Down Electron loses energy to a photon - gives off light Electron goes to a lower energy level 13.6eV 13.6eV losing energy | E | nf 2 ni 2 ee- photon Fluorescence Fluorescent material excited by absorbing light of 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 Fluorescence Fluorescent material excited by absorbing UV photon, then de-excites by emitting two visible photons demo visible photon UV photon e- visible photon 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 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. 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 Protons and Neutrons Proton positive charge +e Neutron zero charge, neutral Both proton and neutron have mass almost 2000 times the electron’s mass 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 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 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!!! Notation C is atomic symbol for carbon pre-subscript is the atomic number 13 6 C7 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) 13 6 C 13 C Strong Nuclear Force Strong nuclear force is what holds the nucleus together, “Nuclear” because it acts between nucleons, protons and neutrons alike; electrons unaffected 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 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! 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 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 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
