What are the component of light? How are the electrons arranged in the atom? What is the relationship between light and the atom? How does light give clues about the structure of the atom? 1
The Electro- and Magnetic- of light Electromagnetic Radiation Light has an electrical component Light has a magnetic component. 2
Light - Electromagnetic Spectrum Visible light is actually superposition of different colors. Colors of light is due to EMR s different wavelength or frequency 3
Electromagnetic Spectrum Spectrum: Visible light is one component of a gamut of radiation. The solar spectrum The electromagnetic spectrum Visible light (enlarged portion) is but a small part of the entire spectrum. The radiation s energy increases from the radio wave end of the spectrum (low frequency, n and long wavelength, l) to the g-ray and (high frequency and short wavelength). 4
Particle -Wave Duality Planck - Einstein : Energy possesses Mass E = hυ E = 1 2 mc2 hυ = 1 2 mc2 hc λ = 1 2 mc2 2hc c 2 λ = m m = 2hc c 2 λ Light: Wave or Particle? Wave Particle Photons Light has Mass Solar sail Footnote: This particle-wave duality bothered Einstein so much he tried to develop a Unified Model... to his death. Recently String Theory has been developed to explain the behavior of matter and energy in more detailed. 5
Matter: Wave-like property Louis DeBroglie (1923), Sorbonne, Paris. Ph.D. Thesis Particle-Wave Duality If energy (radiation) behaves like a stream of particles, then could matter (under appropriate conditions) show wave-like properties Light possesses momentum which can be measured by pressure of light exerted on object. Important Theory because it set the stage for understanding the behavior of the electrons in an atom. Electrons can sometimes behave like a wave. Best describe as a wave in Quantum mechanical Theory of the Atom. 6
Atomic Line Spectrum Light emits light at all wavelength. Excitation of certain elements or the electrical excitation of certain elements give rise to an atomic line spectrum unique to that atom. 7
Hydrogen Atom and Line Spectrum Hydrogen Line Spectrum When hydrogen gas (or other element) receive high energy spark it emits light with specific l -wavelength signature H 2 (g) absorb energy (H-H bond breaks) H atoms 8
Bohr Planetary Model of the H-atom Bohr Atomic model of the Hydrogen atom Electrons follows circular orbits around the nucleus Electrons could have only certain size orbits (quantum condition) electrons are allowed to higher orbit with an input in energy. 9
Bohr Model of the H-atom Model of the Hydrogen atom / Atomic line Spectru What is the origin of the Atomic Line spectrum? 2 1 3 5 6 7 8 4 P 1 2 3 6 7 8... 5 4 Pashen: n f = 3 Balmer: n f=2 Lyman: n f = 1 Lyman: n f = 1 Pashen: n f = 3 Balmer: n f =2 Lyman: n f = 1 Balmer: n f=2 Pashen: n f = 3 Electromagnetic Spectrum 100 nm 400 nm 600 nm 1000 nm 1400 nm Lyman: λ = 95.0 nm (5-1) 97.3 nm (4-1) 102.6 nm (3-1) 121.6 nm (2-1) Balmer: λ = 410 nm (6-2) 434 nm (5-2) 486 nm (4-2) 656 nm (3-2) Paschen: λ = 1005 nm (7-3) 1094 nm (6-3) 1282 nm (5-3) 1875 nm (4-3) 10
Atomic Line Spectrum H-atom Transition of the electron from a high energy level to a lower energy level results in a photon emission which has a frequency related to the energy difference between the transition. The Rydberg equation is an empirical formula that can be used to predict the frequency, wavelength, or energy of the photon emitted upon electron relaxation. Balmer Rydberg 11
Atomic Line Spectrum (2) Emission Lines for various atom is the EMR emission as a result of the electron relaxation from a higher orbital to a lower one. The Rydberg equation only works for the Hydrogen and Hydrogen-like (species with only one electron) however because Bohr model of the atom breaks down when there are more than two electrons. A more sophisticated theory of the atom was needed in order to determine the energy due to electron-electron repulsion. Consequently, the Schrodinger equation to provide a mathematical model of the atom: HY = E Y. Line emission spectra of hydrogen, mercury, and neon. Excited gaseous elements produce characteristic spectra that can be used to identify the elements as well as to determine how much elements is present in a sample. 12
13 Relative Energies for Shells and Orbitals The solution to the Schrodinger Equation lead to Quantum numbers : n, l,,m l, and m s. The following is a model of the atom based on this theory. Relative Energies of the orbitals 8 s p d f 7 6 5 4 3 2 1 Erwin Schrodinger Erwin Schrödinger (1887-1961) was the only son of well-educated parents. His father owned an oil cloth factory and was an amateur painter and botanist. Erwin was taught at home, by tutors and parents, until he was 11. He then attended school to prepare for university. Schrödinger began to think about explaining the movement of an electron in an atom as a wave. By 1926 he published his work, providing a theoretical basis for the atomic model that Niels Bohr had proposed based on laboratory evidence. The equation at the heart of his publication became known as Schrödinger's wave equation. This was the second theoretical explanation of electrons in an atom, following Werner Heisenberg's matrix mechanics. Many scientists preferred Schrödinger's theory since it could be visualized, while Heisenberg's was strictly mathematical. A split threatened among physicists, but Schrödinger soon showed that the two theories were identical, only expressed differently.
Chemistry of Fireworks 14 When most of us think "fireworks," we think of brilliant bursts of light and color we've seen paint a night sky. But such bursts are merely the spectacular end of fireworks that likely took centuries of experience, weeks of planning, and hours of painstaking labor to fashion and fire. In this feature, pull back the wrapping on a typical aerial display shell and see what it looks like before its glorious denouement in the dark. http://www.pbs.org/wgbh/nova/fireworks/
Summary Light travel through space as a wave of radiation energy. The crest-to-crest distance between waves is the wavelength, and the number of cycles completed in a second is the frequency. In 1900 Max Planck introduced the quantum concept. When an object radiates light, it releases a unit of radiation energy called a photon. In 1913 Niels Bohr suggested that electrons travel in curricular orbits about the nucleus. The electron possesses a specific energy and it is said to occupy an energy level. If an electron changes orbital in the Bohr model, there is a quantum energy change. The line emission line spectrum results from electrons dropping from higher energy level to lower energy levels. Each time an electron drops, a proton of light is released whose energy correspond to the difference in energy between the two levels. In the 1920's our understanding of electrons in atoms became very sophisticated. It was proposed that the energy of electrons can be known only in terms of its probability of being located some where within the atom. The description gave rise to the Quantum mechanical atom. A location within the atom where there is a high probability of finding an electron having certain energy is called an orbital. 15
Optional Assignment: Development of the Modern View of the Atom & Quantum Mechanics 1 Describe electromagnetic radiation, some examples and its properties. What is the speed of light? Who said the speed of light is constant. What is another term for light? (Hint it means discrete packets 2 What is the relationship between the frequency, wavelength, energy and color of light Which light has more energy? Blue photons of light or Green photons of light? Which wavelength of light has more energy, 800nm or 500nm light? 3 What is the first law of Thermodynamics? Combine with the Lavoisier's law, what do these two law say about our universe? Restate both of these Laws 4 How does light interact with the electrons in an atom? What condition must occur if light is to be absorb by an atom? Is light energy destroyed? Upon absorption of light by an atom, what happens to the electrons in the valence shell? 5 Why are photons emitted by atoms after when an electron is tired of being excited? Why do elements have unique atomic line signature? How do the wavelength of the light from the atomic line spectrum relate to the electrons in an atom? Do elements in the same family have the same atomic line spectrum? 6 Explain how the architect of electronic structure of an atom relate to the atomic line spectrum 16 Website Reference: www.whatis.com/electrum.htm www.lbl.gov/microworlds/alstool/emspec/emspec.html www.research.umbc.edu/~budzicho/chp9note1.html www.library.advanced.org/3659/thermodyn/intro.htm www.ronkurtus.com/physcien/thermodyn.htm http://library.advanced.org/16468/hist-c1.htm http://home.earthlink.net/~ssbeaton/addlinfo/atomicspectra.html http://www.hh.se/staff/piku/rydberg/rydberglifework.html http://w3.nai.net/~bobsalsa/tutorial.htm http://www.watertown.k12.wi.us/hs/teachers/buescher/atomtime.html http://gibbs.lgc.peachnet.edu/chm101/atomtheo/index.htm