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Chapter 8

Lecture and Textbook Collaborated Notes - Chapter 8 - CHEM 1A03

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McMaster University
Aadil Merali Juma

Chem 1A03 Chapter 8: Electrons in Atoms 8.1 Electromagnetic Radiation  Electromagnetic Radiation: A form of energy transmission in which electric and magnetic fields are propagated as waves through empty space (a vacuum) or through a medium (such as glass)  Wave: A disturbance that transmits energy through space or a material medium o Crests – high points o Troughs – low points o Amplitude – maximum distance from the center line, above or below o Wavelength – λ lambda; the distance between two successive crests (or the bottoms of two troughs)  SI unit: m (meter) o Frequency – ν nu; the number of crests or troughs that pass through a given point per unit of time  Time , or s (per second)  SI unit: Hz (Hertz) o Speed of Wave – C; product of λ and ν  C = λ x ν  Magnetic field component lies in a plane perpendicular to the electric field component o Figure 8-2, page 296; o Electric Field – the region around an electrically charged particle; presence can be detected by measuring the force on an electrically charged object when it is brought into the field o Magnetic field – found in the region surrounding a magnet  Theory – James Clerk Maxwell in 1865 o Electromagnetic radiation is produced by an accelerating electrically charged particle o Eg/ Radio waves – are a form of electromagnetic radiation produced by causing oscillations (fluctuations) of the electric current in a specially designed electrical circuit o Eg/ Visible light – the accelerating charged particles are the electrons in atoms or molecules  Speed of light – electromagnetic radiation has a constant speed in a vacuum; 2.99792458 x 10 m s An Important Characteristic of Electromagnetic Waves  Constructive Interference – the addition of waves  In Phase – where two waves crests, or two waves troughs coincide and create the highest crest or lowest trough  Out of Phase – where one waves crest and one waves trough coincide and the waves cancel each other out causing the wave to be flat  Destructive Interference – the cancelation of waves  Diffraction – the dispersion of different wavelength components of a light beam through the interference produced by reflection from a grooved surface Chem 1A03 The Visible Spectrum  Refraction – the bending of light; occurs when it passed from one medium to another Suns Emission Spectrum Earth’s Protective Shield Ozone  Ozone and UV Exposure o Approximately 90% UVB radiation (280-320 nm) from the sun is selectively absorbed by photolysi3 of O in the stratosphere, with peak at approx. 20km from Earths surface o Lover O3levels increase the transmittance of UVB radiation to earth – Montreal Protocol: 1987  Biological Impacts o UV B radiation can ionize biological molecules (DNA, Protein) o Chronic exposure to UVB rays increases chance of skin cancer, cataracts and genetic mutations o Body’s response: Produce melanin (dark pigment) to filter UVB radiation  Tan is melanin; it absorbs light, prevents UV from entering the bodies biological molecules  Chemistry of Sunscreen o Propose alternative ways to decrease exposure to UC radiation o Use of sunscreen/cosmetics containing UV absorbing chemicals 8.2 Atomic Spectra  Discontinuous Spectrum – is observed if the source of a spectrum produces light having only a relatively small number of wavelength components  Atomic Spectra - The emitted light produces a spectrum consisting of only a limited number of discrete wavelength components, observed as colored lines with dark spaces between them o Eg/ If the light source is an electric discharge passing through a gas, only certain colors are seen in the spectrum  Figure 8-8a,b, page 300 o Eg/ If the light source is a gas flame into which an ionic compound has been introduced, the flame may acquire a distinctive color indicative of the metal ion present  Figure 8-8c-e, page 300  Production of the line spectrum o Figure 8-9, page 301 o Light source is a lamp containing helium gas at a low pressure Chem 1A03 o When an electric discharge is passed through the lamp, the elements atoms absorb energy, which then emit as light o The light is passed through a narrow slit and them dispersed by a prism o The colored components of the light are detected and recorded on photographic film o Each wavelength appears as a thin line  Robert Bunsen and Gustav Kirchhoff developed the first spectroscope and used it to identify elements  Johann Balmer deduced a formula for the wavelengths of spectral lines o ν = 3.2881 x 10 s (1/2 – 1/n ) o ν = the frequency of the spectral line o n must be an integer greater than 2  The fact that the atomic spectra consists of only limited numbers of well-defined wavelength lines provides a great opportunity to learn about the structures of atoms o Suggests that only a limited number of energy values are available to excited gaseous atoms 8.3 Quantum Theory  Blackbody radiation – light emission from heated solids  Classical theory – predicts that the intensity of the radiation emitted would increase indefinitely o Figure 8-11,page 302 o No limitation on the amount of energy as system may possess  Quantum Theory – energy, like matter, is discontinuous o Proposed by Max Planck o Limits energy to a discrete set of specific values o Quantum of energy – the difference between any two allowed energies of a system  Model used for the emission of electromagnetic radiation – a group of atoms on the surface of the heated object oscillating together with the same frequency o ε = nhν where ε = Energy n is a positive integer ν = oscillator (group of atoms) frequency h = 6.62607x10-3J s (Planck’s constant)  The energy of a quantum of electromagnetic radiation is proportional to the frequency of the radiation o Planck’s Equation: E = hν The Photoelectric Effect  Heinrich Hertz, 1888, discovered that when light strikes the surface of certain metals, electrons are ejected  Electron emission only occurs when the frequency of the incident light exceeds a particular threshol0 value (ν ) o The number of electrons emitted depends on the intensity of the incident light o The kinetic energies of the emitted electrons depend on the frequency of the light  Einstein, 1905, proposed that electromagnetic radiation has particle-like qualities, these particles of light called photons, have a characteristic energy give by Planck’s equation  Particle Model o A photon of energy hν strikes a bound electron, which absorbs the photon energy Chem 1A03 o If the photon energy, hν, is greater than the energy binding the electron to the surface (quantity known as work function), a photoelectron is liberated (photon Threshold Energy of metal  e is ejected with kinetic energy) o Thus – the lowest frequency light producing the photoelectric effect is the threshold frequency, and any energy in excess of the work function appears as kinetic energy in the emitted photoelectrons  Key Observations o Work function (threshold energy, hν ) of the metal must be - 0 overcome for e to be emitted o When incident light frequency exceed a threshold value (ν 0 e are emitted o # of electrons (current) emitted depends on light intensity (# of photons) o Kinetic Energy of emitted electrons depends on Energy of light o E (incident light) = Threshold E + KE of e-  Figure 8-12a, page 304; The Photoelectric Effect o First circuit  Light (hν) is allowed to shine on a piece of metal in an evacuated chamber  The electrons emitted by the metal (photoelectrons) travel to the upper plate and complete an electric circuit set up to measure the photoelectric current through an ammeter  Figure 8-12b, page 304; The Photoelectric Effect o First circuit  Illustrates the variation of the photoelectric currentp I , through an ammeter as the frequency (ν), and the intensity of the incident light is increased  No matter how intense the light, no current flows if the frequency is below the threshold frequency, ν 0 and no photoelectric current is produced  No matter how weak the light, there is a photoelectric current if ν > 0  The magnitude of the photoelectric current is directly proportional to the intensity of the light, so that the number of photoelectrons increases with the intensity of the incident light  We can associate light intensity with the number of photons arriving at a point per unit time  Figure 8-12a, page 204; The Photoelectric Effect o Second circuit  Measures the velocity of the photoelectrons  A potential difference (voltage) is maintained between the photoelectric metal and an open-grid electrode placed below the upper plate  For electric current to flow, electrons must pass through the openings in the grid and onto the upper plate  The negative potential on the grid acts to slow down the approaching electrons  As the potential difference between the grid and the emitting metal is increased, a point is reached at which the photoelectrons are stopped at the rid and the current ceases to flow through the ammeter  At the stopping voltage (the potential difference when the current ceases to flow), the kinetic energy of the photoelectrons has been converted to potential energy, expressed through the following equation 2 ½ mu = eV s m = mass of electron u = speed of electron e = charge of electron  V ss proportional to the frequency of the incident light, but independent of the light intensity, is below the threshold frequency, ν0, no photoelectric current is produced  At frequencies greater than ν0, the empirical question for the stopping voltage is V s k(ν – ν0) Constant k is independent of the metals used Chem 1A03 ν0varies from one metal to another  There is no relation between V snd light intensity – but photoelectric current,pI , is proportional to the intensity of light o The work function is a quantity of work and of energy o One way to express this quantity is as the product of Planck’s constant and the threshold frequency E = hν o Another way is as the product of the charge on the electron, e, and the potential0,V that has been overcome in the metal: E = eV 0 o Thus, the threshold frequency for the photoelectric effect is given by: ν0= eV 0 h o Since the work function (eV 0 is a characteristic of the metal used in the experiment, the ν0 is also characteristic of the metal o When a photon of energy hν strikes an electron, the electron overcomes the work function eV0 and is liberated with kinetic energy (1/2 mu ) We have ½ mu + eV –0hν 2 Which gives eVs= ½ mu = hν – eV 0 Which is identical to the empirically determined equation for Vswith k = h/e when hν 0 Photons of Light and Chemical R
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