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Unit 4.4 Bohr’s Model of the Atom Teacher: Dr. Van Der Sluys Objectives • Light and energy – White light – Colors • Line spectra • Bohr Model of the atom – Spectral emission lines Newton and Light • Isaac Newton found that white light could be separated into colors and that the colors could be recombined to form white light again. 1 Electromagnetic Spectrum E = hc λ Gas Discharge Tubes • Similar to Thompson’s experiment, a gas discharge tube has a cathode and an anode. When a large voltage is applied electrons jump off the cathode and collide with the gases in the tube. Line Spectra of Elements • When the light from a discharge tube is passed through of prism (or grating) the wavelengths of light can be separated. Hydrogen Helium 2 Bohr Model • • • • The Bohr model of the atom is similar to the solar system. The nucleus is at the center of the atom and contains the vast majority of the mass of the atom. Electrons orbit the nucleus at fixed distances (energy levels). These orbits are assign integer values (n = 1, 2, 3, etc). These numbers are called the principle quantum number. Electrons can jump from an occupied energy level to an empty level by absorbing or emitting energy (light). Energy States • The lowest energy situation for an atom is to have as many electrons as possible in orbits as close to the nucleus as possible. This is called the ground state. • Excited states are any other situation in which one or more electrons are located in orbits further from the nucleus than in the ground state. • The energy levels are said to be quantized with a specific amount of energy being required for each level. • The ionization energy is the energy required to have the electron jump completely out of the atom. The Bohr Model and Spectral Lines • • • The emission lines for hydrogen in the visible portion of the spectrum are due to the electron jumping from levels where n ≥ 3 to n = 2 (Balmer series). The Lyman series occurs in the ultraviolet region of the spectrum and are due to the electron jumping from levels of n ≥ 2 to n = 1. The Paschen series occurs in the infrared portion of the spectrum and is due to the electron jumping from n ≥ 4 to n = 3. 3 Elements Other than Hydrogen • The spacing between energy levels for atoms other than hydrogen are not as systematic and result in much more complicated line spectra. • Each element has a unique line spectrum and the intensity of a line is proportional to the amount of that element in the sample. The Sun and the Stars • The sun and other stars are powered by nuclear reactions. • The sun gives off light with intensities corresponding to spectral lines of various elements, predominantly hydrogen helium and a variety of other elements. • The Doppler shift can be used to determine if stars are moving towards or away from earth. Most stars appear to be moving away from earth. Conclusion: The universe is expanding. Photons and the Photoelectric Effect • Energy is quantized. • Light is a form of energy. • Therefore, light must be quantized • These smallest pieces of light are called photons. • Photoelectric effect? When photons of the correct energy strike the surface of a metal it can cause ionization. 4