Outline Chapter 9 The Atom Photons Photons The Photoelectron Effect Photons Photons

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1 Outline Chapter 9 The Atom 9-1. Photoelectric Effect 9-3. What Is Light? 9-4. X-rays 9-5. De Broglie Waves 9-6. Waves of What? 9-7. Uncertainty Principle 9-8. Atomic Spectra 9-9. The Bohr Model Electron Waves and Orbits The Laser Quantum Mechanics Quantum Numbers Exclusion Principle All the quanta associated with a particular frequency of light have the same energy. The equation is E = hf where E = energy, h = Planck's constant (6.63 x J s), and f = frequency. Electrons can have only certain discrete energies, not energies in between. Max Planck ( ) Max Planck in 1900 stated that the light emitted by a hot object (black body radiation) is given off in discrete units or quanta. The higher the frequency of the light,the greater the energy per quantum The Photoelectron Effect The photoelectric effect is the emission of electrons from a metal surface when light shines on it. The discovery of the photoelectric effect could not be explained by the electromagnetic theory of light. Albert Einstein developed the quantum theory of light in The system shown here detects people with fevers on the basis of their infrared emissions, with red indicating skin temperatures above normal. In this way people with illnesses that may be infectious can be easily identified in public places. Einstein expanded Planck's hypothesis by proposing that light could travel through space as quanta of energy called photons. Einstein's equation for the photoelectric effect is hf = KE + w. Although photons have no mass and travel with the speed of light, they have most of the other properties of particles. The higher the frequency (or shorter the wavelength) the higher the energy. Albert Einstein ( ) 1

2 Photoelectron(ic) Effect 9-5. De Broglie Waves ffect/photo.htm Louis de Broglie( ) In 1924, the French physicist Louis de Broglie proposed that moving objects behave like waves; these are called matter waves. The de Broglie wavelength of a particle of mass m and speed v is λ = h / mv What is light? Light exhibits either wave characteristics or particle (photon) characteristics, but never both at the same time. The wave theory of light and the quantum theory of light are both needed to explain the nature of light and therefore complement each other De Broglie Waves An electron Microscope and a micrograph of bacteriophage viruses approximately 1 µm across X-rays 9-6. Waves of What? Wilhelm Roentgen accidentally discovered x-rays in In 1912, Max von Laue showed that x-rays are extremely high frequency em waves. X-rays are produced by high energy electrons that are stopped suddenly; the electron KE is transformed into photon energy. The quantity that varies in a matter wave is called the wave function (ψ). The square of the wave function (ψ 2 ) is called the probability density. For a given object, the greater the probability density at a certain time and place, the greater the likelihood of finding the object there at that time. The de Broglie waves of a moving object are in the form of a group, or packet, of waves that travel with the same speed as the object. Wilhelm Roentgen ( ) 2

3 9-7. The Uncertainty Principle Emission Line Spectra of Some Common Elements The uncertainty principle states that it is impossible to know both the exact position and momentum of a particle at the same time. The discoverer of the uncertainty principle was Werner Heisenberg. The position and motion of any object at a given time can only be expressed as probabilities. Werner Heisenberg ( ) 9-8. Atomic Spectra 9-8. Atomic Spectra A spectroscope is an instrument that disperses the light emitted by an excited gas into the different frequencies the light contains. refraction Wavelengths separated An absorption spectrum consists of the various frequencies absorbed by a substance when white light is passed through it. The frequencies in the spectrum of an element fall into sets called spectral series. Seen here is the spectral series of hydrogen Atomic Spectra 9-9. The Bohr Model An emission spectrum consists of the various frequencies of light given off by an excited substance. Below is the part of the emission spectra of sodium. The Niels Bohr model of the atom, proposed in 1913, suggested that an electron in an atom possesses a specific energy level that is dependent on the orbit it is in. An electron in the innermost orbit has the least energy. He predicted that the distance from the proton to the electron in a hydrogen atom was about 0.89Å. Niels Bohr ( ) 3

4 9-9. The Bohr Model Electron Waves and Orbits Electron orbits are identified by a quantum number n, and each orbit corresponds to a specific energy level of the atom. An atom having the lowest possible energy is in its ground state; an atom that has absorbed energy is in an excited state. An electron can circle a nucleus only in orbits that contain a whole number of de Broglie Wavelengths. The quantum number n of an orbit is the number of electron waves that fit into the orbit Electron Waves and Orbits When an electron "jumps" from one orbit (energy level) to another, the difference in energy between the two orbits is hf, where h is Planck s constant and f is the frequency of the emitted or absorbed light Electron Waves and Orbits Electrons seemed to be locked into these wave patterns around the nucleus. Origin of Absorption Spectra The Laser The word laser comes from light amplification by stimulated emission of radiation. 4

5 9-11. The Laser Holograms Excitation Methods include electrical charge, light (below), and chemical reaction. 3D pictures made by Lasers using the interference pattern between reflected laser light from the surface of an object and the undisturbed laser light reflected from a mirror. The Interference pattern is recorded on film. The developed film can then be used by a laser to recreate the image in 3D The Laser Lasers are used in light shows and eye surgery The Laser Holograms are made from laser light without using an image forming device. The image formed becomes 3D. Meet Mika..(Artoolkit) The Laser Holograms are made from laser light without using an image forming device. The image formed becomes 3D Quantum Mechanics Erwin Schrödinger ( ) The theory of quantum mechanics was developed by Erwin Schrödinger, Werner. According to quantum mechanics, the position and momentum of a particle cannot both be accurately known at the same time. Only its most probable position or momentum can be determined. The most probable distance between the proton and electron for a hydrogen atom turns out to be about 0.89Å, the same as Niels Bohr. 5

6 9-13. Quantum Numbers Quantum Numbers 1. The principal quantum number n governs the electron's energy and average distance from the nucleus. 2. The orbital quantum number l determines the magnitude of an atomic electron's angular momentum. 3. The magnetic quantum number m l specifies the direction of an atomic electron's angular momentum. 4. The spin magnetic quantum number m s of an atomic electron has two possible values, +1/2 or -1/2, depending on whether the electron aligns itself along a magnetic field (+1/2) or opposite to the field (-1/2). 3py 2py 1s 2s 2px 2pz3pz 3d 3s 3px Quantum Numbers principal quantum number n orbital quantum number l 1,2,3,.. 0,1,2,.n-1 magnetic quantum number m -l to +l for n=2-2,-1,0,1,2 spin magnetic quantum number +½ or ½ spin Its like your address. To find where you are you need to know 4 things: state, city, street, house #. To know where or what state the electron is in you need to know the four quantum numbers Quantum Numbers The outside of the2s orbital can be seen below. The 2p orbital is in the middle. A combination of the 3 2p orbitals is shown in the movie on the right. Movie Quantum # s are like an Address. What do you need to know to find out where you live? Scanning Tunneling Microscope State City Street House Principle Quantum # (n) Angular Quantum # (l) Magnetic Quantum # (m l ) Spin Quantum # (m s ) 6

7 Scanning Tunneling Microscope Electron Clouds Lecture Quiz 9 1. What did Planck discover? Einstein? 2. What did De Broglie find? 3. What does it mean for the energy of an electron to be quantized? 4. What can we use to see individual atoms? 5. What is a laser? Image Movie The Exclusion Principle The exclusion principle, first proposed by Wolfgang Pauli in 1925, states that only one electron in an atom can exist in a given quantum state. Each atomic electron must have a different set of quantum numbers n, l, m l, and m s. Fig Lecture Quiz 8 Magnetic resonance imaging (MRI) is a method of mapping tissue density based on proton spin that shows the nature of soft tissue better than x-rays. 1. What did Planck discover? Light is given off in discrete energies. Einstein? Light is absorbed in discrete energies. 2. What did De Broglie find? Matter moves in waves 3. What does it mean for the energy of an electron to be quantized? They have discreet energies. 4. What can we use to see individual atoms? The Scanning Tunneling Microscope 5. What is a laser? Light Amplification by Stimulated Emission Radiation and consists of light all of the same wavelength and coherent or in step with each other. 7

8 Lecture Quiz 9 1. What are the four quantum numbers? 2. Is light a particle or a wave? 3. What does it mean for the energy of an electron to be quantized? 4. What does the Uncertainty Principle mean? 5. What is the quantum theory based on, actual movement of the electrons or the probability of where you most likely will find them? Lecture Quiz 9 1. What are the four quantum numbers? Principle, angular, magnetic and spin. 2. Is light a particle or a wave? both 3. What does it mean for the energy of an electron to be quantized? Has discrete energies. 4. What does the Uncertainty Principle mean? You cannot know both the momentum and location of an electron at the same time. 5. What is the quantum theory based on, actual movement of the electrons or the probability of where you most likely will find them? Probability of where you most likely will find them. 8