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CEM 141, notes, week 5

by: Leah DiCiesare

CEM 141, notes, week 5 Cem 141

Leah DiCiesare

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About this Document

These are the notes from week 5, 10/3-10/7, the first week of chapter two. The textbook notes from this week (pg 38-46) are also included in the notes.
General Chemistry
J. Hu
Class Notes
Energy, General Chemistry, light, electromagntic, radiation, photon
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This 6 page Class Notes was uploaded by Leah DiCiesare on Friday October 7, 2016. The Class Notes belongs to Cem 141 at Michigan State University taught by J. Hu in Fall 2016. Since its upload, it has received 8 views. For similar materials see General Chemistry in Chemistry at Michigan State University.


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Date Created: 10/07/16
Lecture Notes: 10/3­10/7  Electromagnetic Radiation (EM Radiation) o Ex. Radio waves, microwaves, infrared, visible, ultraviolet, x­rays, Gamma rays o Light can be described as a wave or a particle (wave­particle duality) o EM radiation can be described as a wave compound of oscillating electric and  magnetic fields. The fields oscillate in perpendicular planes. o Wavelength ( λ is the distance between any two identical points in neighboring  waves, measured in meters o Amplitude is the distance from the average value to the maximum value, the  intensity   o Frequency (v) is the number of oscillations from the average value to the made by the wave per second (Hz) 8 o c=λv ­ c=velocity of light in vacuum (3.0*10 m/s) λ  and v have an inverse relationship o Energy ­ increases as frequency increases (and wavelength decreases)  ­ no a typical  wave (for electromagnetic wave) o ROY G. BIV ­ order of visible light in spectrum (Red, Orange, Yellow, Green, Blue,  Indigo, Violet) R ­ lowest frequency and highest wavelength V ­ highest frequency and lowest wavelength o In order to detect something using EM radiation we must use a wavelength smaller  than the objects we are observing  Properties of a wave o Diffraction ­ a wave will pass through a barrier with a slit and be small and then  spread out again o Diffraction pattern ­ through one barrier slit, strongest intensity in middle ­  through two slits interference patterns with intense bright spots and darks spots that  alternate o Interference ­ if two waves meet in the same space in the same phase and they  join it will have doubles amplitude (brighter light) constructive; destructive if waves  are in opposite phase, they cancel each other out; usually somewhere in between  Photoelectric Effect o Many metals emit electrons when electromagnetic radiation (light) shines on the  surface Lecture Notes: 10/3­10/7 o The light is transferring energy to the electrons at the metal surface ­ where it is  transformed into kinetic energy that gives the electrons enough energy to "leave" the  atoms in metal o Need energy high enough to overcome the electrostatic force between electrons  and nucleus o Evidence  When a short wavelength (high energy) light shines on metal surface  electrons are emitted ­ measureable current  If light intensity is increased ­ more electrons are emitted   There is a threshold frequency below which no electrons are emitted  ­ no  matter how bright (intense) the light is   If light were just a wave ­ increasing the intensity should increase the  energy and eject more electrons ­ no threshold in frequency  Only light above a certain threshold frequency (energy) will result in  ejected electrons  Below the threshold increasing the intensity has no effect   Einstein postulated that light must come in as packets (or particle or  quanta) ­ called photons  Paradigm shift ­ fundamental change in the basic concepts and  experimental practices of scientific discipline  EM Radiation is a Particle o Energy is transferred as a particle (photon) that has a definable energy o Planck­Einstein relation: E=hv (h is Planck's constant)  One photon can interact and eject one electron, if it has enough energy. If  the photon does not have enough energy then no electron is ejected o Planck: light must have quantized energy related with frequency and proposed the equation: E=hv (h=6.626*10 Js)  But failed to provide physical explanation o Einstein: photoelectric effect ­ threshold of frequency (energy)  "photon" (particle) ­ Nobel Price 1921 o Increasing intensity = increasing photon number Decreasing frequency = lower energy of each photon o o Decreasing frequency below threshold = energy in photo is too low  Summary of Electromagnetic Radiation o Can be described as either a particle or a wave These are models not reality  o Truly difficult to imagine these ideas ­ how can one phenomenon be two different  things? o Wave­particle duality is important at very small scales  Matter and energy don't behave the same as in our macroscopic world  Absorption and Emission Spectra o Visible Spectrum: light from sun (white light) can be separated by a prism  (Newton did this first) ­ only a very small part of the EM spectra Lecture Notes: 10/3­10/7 o Atomic Emission Spectrum: light from one particular atom does not contain all  the colors of the spectrum ­has only a few wavelengths ­ use hot gas to test o Atomic Absorption Spectrum: light that is absorbed in an atomized sample of gas  (cold); continuous spectrum with dark lines that show which light was absorbed by  the sample o Emission + Absorption Spectra = continuous spectrum; they are complementary o Each element has characteristic wavelengths that it can absorb or emit o Spectra show light only of specific wavelengths/energies ­ the spectrum of  element is the same whether that element is on Earth, in the Sun, of in a galaxy light  years away  Niels Bohr Model o Electrons move in orbits around nucleus o These orbits have definite energies and are at definite distances from the nucleus o So the energies are quantized o Explained emission and absorption spectra by invoking discrete energy levels ­  characterized by quantum numbers (n)  Has to have the exact same energy to change electron orbit levels or emit  electrons o Photons of electromagnetic energy are emitted or absorbed by atoms as electrons  move from one energy level to another o The energy of the photons corresponds to the difference in energy levels of the  electrons o Only works for hydrogen ­ so it is misleading o Introduced quantized energy levels of electrons  Better to use energy diagrams Lecture Notes: 10/3­10/7   o Each  energy  level  has a  quantum number o The higher the number, the higher the energy o Energy levels are not orbits o Electrons transition between energy levels by absorbing or emitting photons Textbook Notes: 2.1­2.4  2.1 Light and Getting Quantum Mechanical o Evidence that light is both a wave and a particle o James Clerk Maxwell (1831­1879) developed the electromagnetic theory of light,  in which visible light and other waves were viewed in terms of perpendicular electric  and magnetic fields o A light wave can be described by defining its frequency (v) and its wavelength  (λ)  λv =c  c=velocity of light o Max Planck (1858­1947), German physicist  An object heated to a particular temperature emits radiation (infrared)  Studied how the color of the light emitted changed as a function of an  object's temperature  Result known as the ultraviolet catastrophe ­ reproducible data that didn't  follow the theory but also the theory could not be modified to accept this data  Matter absorbs and emits energy only in discrete chunks called quanta;  quanta occurred in multiples of E (energy)=hv; h is Planck's constant and v is  frequency of light  2.2 Taking Quanta Seriously o 1905 Einstein used idea of quanta to explain the photoelectric effect which was  described and patented by Nikola Tesla o Photoelectric effect occurs when light shines on a metal plate and electrons are  ejected, creating a current o There is a threshold wavelength (energy) of light that is a characteristic for the  metal used, beyond which no electrons are ejected  Explained by the idea that light comes in particle form known as photons  which also have a wavelength and frequency o Intensity of light is related to the number of photons that pass by per second o Energy per photon is dependent upon its frequency or wavelength c=3.0*108 o o λv=c; higher frequency (v), shorter wavelengλh ( ) and greater energy per photon  Energy is directly related to frequency but inversely related to wavelength  E=hc/λ o Once the wavelength is short enough, or the energy is high enough to eject  electrons, increasing the intensity of the light now increases the number of electrons  ejected  There has to be enough energy to overcome the attraction between the  electro and the nucleus o All matter has a wavelength (a wavelike property)  Louis de Broglie (1892­1987)λ  =h/mv, mv is the momentum  (mass*velocity)  2.3 Exploring Atomic Organization Using Spectroscopy o What makes rainbows possible is the face that sunlight is composed of photons  with an essentially continuous distribution of visible wavelengths Textbook Notes: 2.1­2.4 o When a dense body, like the sun, is heated  it emits light of many wavelengths ­  visible colors; when a sample of an element or mixture is heated it emits light of only  very particular wavelengths o If white light were to pass through a cold gaseous element the same wavelengths  that were emitted (when it was heated) would be absorbed while all the other  wavelengths would pass through o Emission and absorption wavelengths for each element are unique o Making sense of the Spectra  Niels Bohr (1885­1962) proposed a new model for the atom  1st hypothesis: electrons within an atom can only travel along  certain orbits at a fixed distance from the nucleus, each orbit  corresponding to a specific energy  2nd idea: electrons can jump from one orbit to another, but this  requires the absorption or emission of energy, in the form of a photon  Photon has to be exactly the right amount of energy  Lower, more stable orbits are visualized as being closer to the  nucleus while higher, less stable, and more energetic orbits are further  away  When enough energy is supplied all at once an electron is removed completely, leaving a positively­charged ion  Only could predict emission/absorption spectrum for hydrogen  2.4 Beyond Bohr o Louis de Broglie used Planck's relationship between energy and frequency  (E=hv), the relationship between frequency and wavelength λc= v), and Einstein's  relationship between energy and mass (E=mc ) to derive a relationship between mass  and wavelength for any particlλ ( =h/mv)  At atomic scale, wavelengths associated with particles are similar to their  size o Certainty and Uncertainty  Have to use electromagnetic radiation of a wavelength similar to the size  of an electron to see it  When wavelengths of that small interact with an electron it changes the  electron's position and motion  The uncertainty created by trying to measure where something is gets  greater the closer we get to the atomic­molecular scale  Idea by Werner Heisenberg (1901­1976) ­ Heisenberg Uncertainty  Principle  Can estimate uncertainty usinΔ  mv*Δx>h/2 π Δmv=uncertainty  in momentum of particle,Δ x is uncertainty in position


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