Dr. Jie ZouPHY Chapter 42 Atomic Physics

Dr. Jie ZouPHY Outline Atomic spectra of gases Early models of the atom Bohr’s model of the hydrogen atom

Dr. Jie ZouPHY Atomic spectra of gases (a) Emission line spectra for H, Hg, and Ne. (b) Absorption spectrum for H. Orion Nebula

Dr. Jie ZouPHY “Neon” signs: an application

Dr. Jie ZouPHY Balmer series of hydrogen Johann Jocob Balmer ( ) The empirical equation by Johannes Rydberg ( ): R H : Rydberg constant = x 10 7 m -1. The series limit The measured spectral lines agree with the empirical equation to within 0.1%. The Balmer series of spectral lines for atomic hydrogen.

Dr. Jie ZouPHY Early models of the atom Model of the atom in the days of Newton: Tiny, hard, and indestructible sphere. J.J. Thomson’s model of the atom: Negatively charged electrons in a volume of continuous positive charge. Rutherford’s planetary model of the atom.

Dr. Jie ZouPHY Difficulties with Rutherford’s planetary model Cannot explain the phenomenon that an atom emits (and absorbs) certain characteristic frequencies of electromagnetic radiation and no others. Predication of the ultimate collapse of the atom as the electron plunges into the nucleus.

Dr. Jie ZouPHY Bohr’s model of the hydrogen atom Basic ideas of the Bohr theory of the hydrogen atom: The electron moves in circular orbits around the proton under the electric force of attraction. Only certain electron orbits are stable. When in one of these stationary states, the electron does not emit energy in the form of radiation. Radiation is emitted by the atom when the electron makes a transition from a more energetic initial orbit to a lower- energy orbit. The frequency of the emitted radiation is found from E i –E f = hf. Energy of an incident photon can be absorbed by the atom only if the photon has an energy that exactly matches the difference in energy between an allowed state of the atom and its existing state upon incidence of the photon. The size of an allowed electron orbit is determined by a condition imposed on the electron’s orbital angular momentum: quantization of the orbital angular momentum m e vr=nħ, n = 1,2,3…

Dr. Jie ZouPHY Bohr’s theory of hydrogen atom (cont.) Allowed energy levels (see detailed derivation): a 0 = Bohr radius = ħ 2 /m e k e e 2 = nm r n =n 2 a 0 = n 2 ( nm) Ionization energy: the minimum energy required to ionize the atom in its ground state (to completely remove an electron from the proton’s influence) = 13.6 eV for hydrogen. Emission frequency and wavelength:

Dr. Jie ZouPHY Energy-level diagram for the hydrogen atom

Dr. Jie ZouPHY Quick quiz #1 A hydrogen atom is in its ground state. Incident on the atom are many photons each having an energy of 10.5 eV. The result is that (a) the atom is excited to a higher allowed state (b) the atom is ionized (c) the photons pass by the atom without interaction

Dr. Jie ZouPHY Quick Quiz #2 A hydrogen atom makes a transition from the n = 3 level to the n = 2 level. It then makes a transition from the n = 2 level to the n = 1 level. Which transition results in emission of the longest-wavelength photon? (a) the first transition (b) the second transition (c) neither, because the wavelengths are the same for both transitions.

Dr. Jie ZouPHY Homework Chapter 42, P. 1392, Problems: #5, 6, 8, 12.