Chemistry 141 Monday, October 30, 2017 Lecture 23 Light and Matter Chemistry 11 - Lecture 11 9/30/2009 Chemistry 141 Monday, October 30, 2017 Lecture 23 Light and Matter

The photoelectric effect The experiment: When light is shined on a metal surface under certain conditions, free electrons are ejected frequency (s-1) KE Light source threshold frequency

The photoelectric effect Einstein (1905) proposed that light is quantized – that is, it behaves like particles  = wavelength (m) n = Frequency (Hz, s-1) c = 2.99792458 ×108 m/s (in vac.) h = 6.626×10-34 J·s hν1< hν2 < hν3 hν1 hν1 hν1 hν2 hν2 hν2 hν3

Quantization of energy

Photon energies  = wavelength (m) n = Frequency (Hz, s-1) c = 2.99792458 ×108 m/s (in vac.) h = 6.626×10-34 J·s Earlier, we calculated the wavelength of red light (700 nm). What is the energy of a photon with this wavelength?

The electromagnetic spectrum 400 nm 700 nm Examples of electromagnetic radiation: visible light x-rays microwave ovens radio and TV signals

Light and Matter

Continuous vs. line spectra When white light is dispersed through a prism, the resulting spectrum is continuous When light from a hydrogen lamp is dispersed through a prism, it is observed to consist of discrete lines What wavelengths of light are emitted? Copyright © 2002 by Houghton Mifflin Company. All rights reserved.

The Balmer Series n n (Hz) l (nm) 3 4.5711014 656.3 Johann Balmer worked out that the frequencies of the light emitted by excited H-atoms satisfy the equation below n n (Hz) l (nm) 3 4.5711014 656.3 4 6.1721014 486.1 5 6.9121014 434.0 6 7.3141014 410.2 For n = 3, 4, 5, …

The Balmer Series n n (Hz) l (nm) 3 4.5711014 656.3 Johann Balmer worked out that the frequencies of the light emitted by excited H-atoms satisfy the equation below n n (Hz) l (nm) 3 4.5711014 656.3 4 6.1721014 486.1 5 6.9121014 434.0 6 7.3141014 410.2 For n = 3, 4, 5, … where n2 > n1 RH = 1.096776×107 m-1 (Rydberg’s constant)

The Bohr atom Bohr postulated that electrons traveled in circular orbits Only orbits with certain radii were allowed n can only be an integer!! The energy of an electron in a given orbit is: E depends on n Energy is also quantized Note that E is negative!

Atomic energy levels Energy levels are like a staircase A change from lower to higher energy level corresponds to absorption of energy A change from higher to lower energy level corresponds to emission of energy E=0 energy of level n=4 energy to move between levels energy n=3 “excited states” n=2 n=1 “ground state”

Bohr atom energies Brackett Paschen Energy Balmer Lyman Energy n =  -13.6 eV n = 5 n = 6 Lyman -3.40 eV -1.51 eV -0.85 eV -0.54 eV -0.38 eV 0 eV Balmer Paschen Brackett Energy Bohr atom energies Energy

Example What is the wavelength of the photon absorbed when going from the n=2 energy level to the n=4 energy level?

The Balmer Series n n (Hz) l (nm) 3 4.5711014 656.3 Johann Balmer worked out that the frequencies of the light emitted by excited H-atoms satisfy the equation below n n (Hz) l (nm) 3 4.5711014 656.3 4 6.1721014 486.1 5 6.9121014 434.0 6 7.3141014 410.2 For n = 3, 4, 5, …

Bohr atom: does it work? What next? Made accurate predictions for H and He+ line spectra Theoretical basis of Bohr’s model is questionable There is no physical explanation for why the electron “orbits” are stable Bohr simply threw in idea of “allowed orbits” to classical physics to get agreement with experiments Only works for one-electron atoms Predictions for multielectron atoms were incorrect What next?