Chapter 4.3-4.4 – Characteristics of Atoms and Light CHM1111 Section 04 Instructor: Dr. Jules Carlson Class Time: M/W/F 1:30-2:20 Friday, September 23rd

Housekeeping Textbooks and I clickers are in!!!!! I-Clicker questions will be official starting Monday!!!! Group finds particles moving faster than the speed of light!!!

Units and values for Energy/Wavelength/Frequency Energy is given in J = kg m-2 s-2 Energy can be given per photon (J/photon), per mole (J/mol), or per Einstein (J/Ein) Wavelength is given in nm for electromagnetic radiation of light To remove electrons from metals in Photoelectric effect experiments, = 100 – 400 nm in near UV. Frequency given in cycles s-1 (usually abbreviated to s-1). Values of 1014 – 1015 for light Planck’s constant = 6.626 x 10-34 Js - Units of energy divided by frequency Intensity commonly given as photons per surface area per time (photons cm-2 s-1) or in einsteins (Ein cm-2 s-1)

Photoelectric Effect Problem The binding energy of electrons to a chromium metal surface is 7.21 x 10-19 J. Calculate: The longest wavelength of light that will eject electrons from chromium metal The frequency required to give electrons with kinetic energy of 2.50 x 10-19 J. The wavelength of light for part (b)

I-Clicker Question Which of the following statements is/are NOT true: If emitted light has a frequency of 1.1 x 1015 s-1 and the threshold frequency related to the binding energy for Cs metal is 1.3 x 1015 s-1, electrons will be ejected from Cs. Light of higher frequency has a shorter wavelength. Above the threshold frequency, the number of emitted electrons and the energy per electron increase with light intensity. Both (a) and (c). Both (a) and (b).

Electrons and Light Absorption  

Quantization of Energy   n = 5 n = 4 n = 3 n = 2 n = 1 Ground state

Atomic Spectra Atoms absorb specific and characteristic frequencies/wavelengths of light. Depends upon the energy differences between ground and excited states for the atoms electrons. The pattern of absorbed photons is an absorption spectrum.

Atomic Spectra When a photon is absorbed, it has to have a wavelength or frequency which matches the difference between the energy levels. Sometimes, the electron can drop from a higher energy state to a lower energy state producing a photon - the photon produced also has a wavelength or frequency that matches the difference between the energy levels.

Atomic Spectra Problem What is the energy of a photon needed to excite an electron from the ground state of hydrogen to the n=5 level? Determine the frequency and wavelength that hydrogen atoms emit in transition from the n=5 level to the ground state.

Properties of Electrons Electrons all have the same mass of 9.109 x 10-31 kg and charge of 1.602 x 10-19 C Note 1 Coulomb, C = charge of 6.241 x 1018 electrons Electrons have magnetic properties, due to electron spin. First proposed by De Broglie, electrons also have wave and particle properties like photons.

Momentum of photons