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Chapter 8 - Electrons in Atoms

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CHEM 110
Ariel Fenster

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) One of the four forces Known in Nature Mediators of electromagnetic force are photons :(massless particles of spin one) 1. Electromagnetic force chemistry infinite range F= (k) (q1 2/ r ) (r) (r is a unit vector) F is Faraday (the charge on a mole of electrons) 2. Gravity cosmology infinite range 2 F= (G) (m 1 2 r ) (r) (r is a unit vector) 3. Weak nuclear force nuclear physics within nucleus short range 4. Strong nuclear force nuclear physics - with nucleus short range A wave is a disturbance that transmits energy through space or a material medium. Waves of light are always in a SINUSODIAL MOTION. The maximum height of the wave above the center line or the maximum depth below is called the amplitude (A) The distance between the tops of two successive crests is called the wave lengths Frequency is the number of crests or troughs that pass through a given point per unit of time () Phases: shifting the sine curve to the left o right along the direction of propagation () Nodes: when the wave hits zero along the sinusoidal motion (nodal line is in two dimension) Antinodes: when the waves hit its maximum along the sinusoidal motion Electromagnetic Radiation: An oscillating charge produces EM radiation which displays a frequency equal to the frequency of oscillation. Sound and water waves travel through a medium EM radiation travels through a vacuum at the speed of light and is mediated (transmitted) by photons -1 The SI unit for frequency, s is the hertz (Hz), and the basic SI wavelength unit is the meter (m). The angstrom is not an SI unit. Speed of light is represented by the symbol c (2.998 x10 ms ), and the relationship between speed and frequency and wavelengths is: c= v x monochromatic light: unique frequency/wavelength v = frequency and = nanometers (convert to m) The wavelength of electromagnetic radiation is shorter for high frequencies and longer for low frequencies. Chapter 8 Electrons in Atoms Interference in two overlapping light waves: In constructive interference, the troughs and crests are in step (in phase), leading to addition of the two waves In destructive interference, the troughs and crests are out of step (out of phase), leading to cancellation of two waves The dispersion of different wavelength components of a light beam through the interference produced by reflection from a grooved surface is called a diffraction (ex. A CD) Fourier series: any wave shape can be created by a superposition of many waves Superposition of waves is essential to quantum mechanics/wave mechanics Refraction of light: Light is refracted (bent) as it passes from air into the glass prism, and again as it emerges from the prism into air. INSERT EM SPECTRUM HERE Shorter the wavelength = higher the frequency Visible light = 390 nm to 790 nm All waves have the same speed in a vacuum 8.2 Atomic Spectra Continuous spectrum: when the light being diffracted consists of many wavelength components Discontinuous spectrum: if the source of a spectrum produces light having only a relatively small number of wavelength components These discontinuous spectra are called atomic, or line, spectra Balmer equation: formula for wavelengths of spectral lines v = 3.2881 x10 s (1/4 1/n )2 o v = frequency of the spectra line o n = an integer greater than 2 Line Spectra: emission of electrons in quantum jumps Rydnerg Formula: H is Plancks constant n1and n 2re integers from 1 to infinite RH= 3.2881 x 10 s5 -1 E= hv v= Rh (1/n = 11n ) 22Chapter 8 Electrons in Atoms 8.3 Quantum Theory Intensity does not increase indefinitely: energy, like matter, is discontinuous. Classical physics places no limitations on the amount of energy a system may possess, whereas quantum theory limits this energy to discrete set of specific values The difference between any two allowed energies of a system also has a specific value called a quantum of energy (when energy increases, it increases by a tiny jump or quantum) Plancks assumption: = nhv is energy, n is a positive integer, v is the oscillator frequency and h is Plancks constant (h = 6.62607 x 10 -3J s) Plancks equation: E = hv (J/photon) Higher the frequency, the greater the energy Used to develop relationships among frequencies, wave lengths and energy Blackbody Radiation: first experiment that classical mechanics could not explain; as a blackbody is heated, the atoms start to oscillate faster and faster, and as they do, the frequency of emitted light increases without limit 8.4 Photoelectric Effect Discovered by Heinrick Hertz in 1988, explained by Einstein in 1905; when light strikes the surface of certain metals, electrons are ejected. 1. Light intensity depends on the number of ph
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