che106_lecture14(1)

che106_lecture14(1) - Chemistry 106 Lecture 14 Topic:...

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Unformatted text preview: Chemistry 106 Lecture 14 Topic: Quantum Mechanics Announcements Exam #2 (Chapters 4,5,6) will be held NEXT MONDAY October 31st During normal class hours (5.15-6.35) at Stolkin A study guide for Exam #2 was posted on Blackboard on October 18th Prior to the work of Niels Bohr (1922 Nobel Prize in Physics) , the stability of the atom could not be explained using the then-current theories. In 1913, using the work of Einstein and Planck, he applied a new theory to the simplest atom, hydrogen . Before looking at Bohrs theory, we must first examine the line spectra of atoms. The Bohr Theory of the Hydrogen Atom When a heated metal filament (light bulb) emits light, we can use a prism to spread out the light to give a continuous spectrum-that is, a spectrum containing light of all wavelengths. The light emitted by a heated gas , such as hydrogen, results in a line spectrum- a spectrum showing only specific wavelengths of light. Atomic Line Spectra Neon For atoms and molecules one does NOT observe a continuous spectrum, as one gets from a white light source. Only a line spectrum of discrete wavelengths is observed. Atomic Line Spectra Dispersion of white light by a prism Dispersion of atomic emission light by a prism In 1885, J. J. Balmer showed that the wavelengths, , in the visible spectrum of hydrogen could be reproduced by a simple formula. The known wavelengths of the four visible lines for hydrogen correspond to values of : n = 3, n = 4, n = 5, and n = 6. The Bohr Theory of the Hydrogen Atom - =- 2 2 1 7 1 1 2 1 m 10 1.097 n Bohrs Postulates Bohr set down postulates to account for (1) the stability of the hydrogen atom and (2) the line spectrum of the atom. 1. Energy level postulate : An electron can have only specific energy levels in an atom (quantized). 2. Transitions between energy levels : An electron in an atom can change energy levels by undergoing a transition from one energy level to another. The Bohr Theory of the Hydrogen Atom Bohrs Postulates Bohr derived the following formula for the energy levels of the electron in the hydrogen atom. n is the principal quantum number. Rh is the Rydberg constant (expressed in energy units) with a value of 2.18 x 10-18 J. The Bohr Theory of the Hydrogen Atom atom) H (for ..... 3 2, 1, n n R E 2 h =- = Energy- level diagram for the electron in the hydrogen atom. atom) H (for ..... 3 2, 1, n n R E 2 h =- = Bohrs Postulates When an electron undergoes a transition from a higher energy level to a lower one, the energy is emitted as a photon....
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che106_lecture14(1) - Chemistry 106 Lecture 14 Topic:...

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