PH 103 Dr. Cecilia Vogel Lecture 21

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

Transition Up  Electron absorbs energy  perhaps from a photon  goes to a higher energy level e-e- photon e-e-

Transition Down  Electron loses energy to a photon - - gives off light  Electron goes to a lower energy level  losing energy e-e- e-e- 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 e-e- UV photon 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 = m  compare to size of atom = several Å = 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=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  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 = = 7)  Don’t need to give Z, can find it  in the periodic table (Carbon is Z=6)

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)  m nucleus c 2 < Zm p c 2 + Nm n c 2  m nucleus < Zm p + Nm n  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(m p +m e )c 2 + Nm n c 2 ] – m atom c 2  BE =[Z(m 1H )c 2 + Nm n c 2 ] – m atom c 2

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 107 Ag is u  What units do we get when we do mc 2 ?  Example mass energy of 107 Ag  = mc 2 = ( u) c 2.  = uc 2  Do not leave these units in answers.  Convert: 1 uc 2 =931.5MeV  uc 2 = MeV