CHEMISTRY & YOU What gives gas-filled lights their colors? Chapter 5 Electrons In Atoms 5.1 Revising the Atomic Model 5. Electron Arrangement in Atoms 5.3 Atomic and the Quantum Mechanical Model An electric current passing through the gas in each glass tube makes the gas glow with its own characteristic color. 1 3 4 The frequency (ν) number of wave/cycles to pass a given point per unit of time. The SI unit of cycles per second is called the hertz (Hz). (c) the speed of light in a vacuum = 3.00 X 10 8 m/s ALL electromagnetic radiation c = λν 5 6 1
The frequency and wavelength are inversely proportional Sun and incandescent light bulbs emit white light continuous range of wavelengths and frequencies. Prisms separate different wavelengths into a spectrum of colors. In the visible spectrum Red light has the longest wavelength and the lowest frequency. 7 8 Electromagnetic Spectrum Low energy (λ = 700 nm) High energy (λ = 380 nm) Frequency ν (s -1, Hz) 3 x 10 6 3 x 10 1 3 x 10 Atomic When atoms absorb energy, their electrons move to higher energy levels. Electrons release energy/emit light when returning to lower energy levels. 10 10-8 10-14 Wavelength λ (m) 9 10 Atomic A prism separates light into the colors it contains. White light produces a rainbow of colors. Atomic Light from a helium lamp produces discrete lines. 11 Light bulb Slit Prism Screen 1 Helium lamp Slit Prism Screen
Sample Problem 5. Atomic Energy absorbed by an electron = energy of the light emitted by the electron as it drops back to its original energy level. Atomic emission spectrum : Wavelengths of the spectral lines characteristic of the elemen No two elements have the same emission spectrum. Calculating the Wavelength of Light Calculate the wavelength of the yellow light emitted by a sodium lamp if the frequency of the radiation is 5.09 10 14 Hz (5.09 10 14 /s). 13 14 Sample Problem 5. Calculate Solve for the unknown. Substitute the known values for n and c into the equation and solve. What is the frequency of a red laser that has a wavelength of 676 nm? λ = 5.89 x 10 7 m ν = 4.43 x 10 14 s -1 15 16 Planck, Einstein, DeBroglie, & Heisenberg Classical physics (treating light as a wave) could not explain all of the observations being made of e- and light The Quantization of Energy Max Planck Could predict changes in color of heated objects if he assumed energy was traveling in little packets (quanta) Beginning of treating light as a particle, instead of a wave 17 18 3
The Quantization of Energy E ν or E = hν h= 6.66 x 10 34 J s Planck s constant Based on the value of Planck s constant, what can you say about the size of these packets of energy? Albert Einstein used Planck s quantum theory to explain the photoelectric effect. electrons are ejected when light shines on a metal. 19 0 Wikipedia No electrons are ejected because the frequency of the light is below the threshold frequency. If the light is at or above the threshold frequency, electrons are ejected. If the frequency is increased, the ejected electrons will travel faster. The photoelectric effect could not be explained by classical physics. Classical physics treated light as only a form of energy/continuous wave. According to this, an electron in a metal should eventually collect enough energy to be ejected, even if exposed to low energy light (assumed light was continuous). 1 To explain the photoelectric effect, Einstein proposed that light could be described as quanta of energy that behave as if they were particles. Photons An Explanation of Atomic Spectra An Explanation of Atomic Spectra The light emitted by an electron moving from a higher to a lower energy level has a frequency directly proportional to the energy change of the electron. 3 4 4
In the hydrogen spectrum, which of the following transitions produces a spectral line of the greatest energy? A. n = to n = 1 B. n = 3 to n = C. n = 4 to n = 3 In the hydrogen spectrum, which of the following transitions produces a spectral line of the greatest energy? A. n = to n = 1 B. n = 3 to n = C. n = 4 to n = 3 5 6 How does quantum mechanics differ from classical mechanics? Given that light behaves as waves and particles, can particles of matter behave as waves? Louis de Broglie referred to the wavelike behavior of particles as matter waves. All matter has properties of waves, but as mass increases the wave properties become less important in describing the behavior of the object. http://www.nobelprize.org/ nobel_prizes/physics/laureates/ 199/broglie-bio.html 7 8 Classical mechanics adequately describes the motions of bodies much larger than atoms Quantum mechanics describes the motions of subatomic particles and atoms as waves. The Heisenberg Uncertainty Principle It is impossible to know both the velocity and the position of a particle at the same time. Before collision: A photon strikes an electron during an attempt to observe the electron s position. After collision: The impact changes the electron s velocity, making it uncertain. Why can we easily measure the velocity and location of cars, etc.? 9 30 5