Light

Quantum Theory Max Planck (1900) – Observed - emission of light from hot objects – Concluded - energy is emitted in small, specific amounts (quanta) – Quantum - minimum amount of energy change Courtesy Christy Johannesson Max Planck

Continuous vs. Quantized Energy Energy A B Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 330 continuous quantized

Quantum Theory Planck (1900) vs. Classical Theory Quantum Theory Courtesy Christy Johannesson

Light Particles – photons (discrete bundles of energy) Light is emitted when electrons return to ground state from an excited state Waves – Frequency, wavelength Wave-Particle duality

Wave-Particle Duality JJ Thomson won the Nobel prize for describing the electron as a particle. His son, George Thomson won the Nobel prize for describing the wave-like nature of the electron. The electron is a particle ! The electron is an energy wave!

The Wave-like Electron Louis deBroglie The electron propagates through space as an energy wave. To understand the atom, one must understand the behavior of electromagnetic waves.

Formation of Light Nucleus e e Lithium Atom + Ground State e e Excited State e Electron Returns to Ground State Light is given off e Ion is formed Li  e + Li 1+ hv n = 1 n = 2 n = 3 n = 4 n = 5 n = 6 n = 7 Nucleus

e-e- e-e- Ground state Excited state Electrons can only be at specific energy levels, NOT between levels.

Energy Level Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 329 A B CD Ground state Energy Four excited states

An Excited Lithium Atom Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 326 Photon of red light emitted Li atom in lower energy state Excited Li atom Energy

Humor Question: Why does hamburger have lower energy than steak? Answer: Because it’s in the ground state.

Visible Spectrum of Light PRISM Slit Ray of White Light Waves 1 / 33,000 ” long Waves 1 / 70,000 ” long R ed O range Y ellow G reen B lue I ndigo V iolet

The Electromagnetic Spectrum AM radio Short wave radio Television channels FM radio Radar Microwave Radio Waves Gamma Rays X- Raysinfrared Increasing photon energy Increasing frequency Decreasing wavelength Red Orange Yellow Green Blue Indigo Violet UV Rays VisibleLightVisibleLight R O Y G B I V HIGHENERGYHIGHENERGY LOWENERGYLOWENERGY

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Common wavelength units for electromagnetic radiation Picometer pm Gamma ray Ångstrom Å X-ray Nanometer nm X-ray Micrometer  m Infrared Millimeter mm Infrared Centimeter cm Microwave Meter m 10 0 Radio Unit Symbol Wavelength, (m) Type of Radiation Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

Red and Blue Light Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 325 Photons - particle of light that carries a quantum of energy

Atomic Spectrum How color tells us about atoms

Prism White light is made up of all the colors of the visible spectrum. Passing it through a prism separates it. Author: Thomas V. Green Jr.

If the light is not white By heating a gas or with electricity we can get it to give off colors. Passing this light through a prism does something different. Author: Thomas V. Green Jr.

Atomic Spectrum Each element gives off its own characteristic colors. Can be used to identify the atom. How we know what stars are made of. Author: Thomas V. Green Jr.

Bohr Model electrons exist only in orbits with specific amounts of energy called energy levels Therefore… electrons can only gain or lose certain amounts of energy only certain photons are produced Courtesy Christy Johannesson

Bohr Model Energy of photon depends on the difference in energy levels Bohr’s calculated energies matched the IR, visible, and UV lines for the H atom Courtesy Christy Johannesson nucleus

Other Elements Each element has a unique bright-line emission spectrum. i.e. “Atomic Fingerprint” Helium z Bohr’s calculations only worked for hydrogen!  Courtesy Christy Johannesson

Emission Spectra excited gas spectroscope

Bohr’s Experiment Kelter, Carr, Scott, Chemistry A Wolrd of Choices 1999, page 76 Animation by Raymond Chang – All rights reserved.

Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. (a) Electronic absorption transition (b) H 2 emission spectrum (top), H 2 absorption spectrum (bottom)

Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. (a) Electronic absorption transition (b) H 2 emission spectrum (top), H 2 absorption spectrum (bottom) Lower-energy orbit Higher-energy orbit Photon e-e- e-e-

continuous spectrum absorption spectrum emission spectrum hot source gas absorption spectrum emission spectrum

Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. (ultraviolet) (visible) (infrared) HYDROGEN SPECTRAL LINES

Hydrogen Spectral Lines A B C D E F Lyman series (UV) A B C D E Balmer (Visible) A B C D Paschen (IR) E1E1 E2E2 E3E3 E4E4 E5E5 E6E6 Energy Bohr’s model of the atom accounted mathematically for the energy of each of the transitions shown. IR region UV region 656 nm 486 nm 434 nm 410 nm Davis, Metcalfe, Williams, Castka, Modern Chemistry, 1999, page 97 ionization

Emission Spectrum of Hydrogen 1 nm = 1 x m = “a billionth of a meter” 410 nm434 nm486 nm656 nm

Emission Spectrum of an Element 1 nm = 1 x m = “a billionth of a meter” 410 nm434 nm486 nm656 nm 1 nm = 1 x m = “a billionth of a meter”

Continuous and Line Spectra 4000 A o A o light Na H Ca Hg

Flame Emission Spectra Photographs of flame tests of burning wooden splints soaked in different salts. methane gas wooden splintstrontium ioncopper ionsodium ion calcium ion

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Flame Tests for Certain Metals BORAX BEAD TESTS FOR CERTAIN METALS (All beads formed in the oxidizing flame)

Fireworks

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Common chemicals used in the manufacture of fireworks Ammonium perchlorate aluminumBlue flame: copper carbonate, copper sulfate, or copper oxide Barium chlorate antimony sulfideRed flame: strontium nitrate or strontium carbonate Barium nitrate charcoalWhite flame: magnesium or aluminum Potassium chlorate magnesiumYellow flame: sodium oxalate or cryolite (Na 3 AlF 6 ) Potassium nitrate sulfurGreen flame: barium nitrate or barium chlorate Potassium perchlorate titaniumWhite smoke: potassium nitrate plus sulfur Strontium nitrateColored smoke: potassium chlorate and sulfur, plus organic dye Whistling noise: potassium benzoate or sodium salicylate White sparks: aluminum, magnesium, or titanium Gold sparks: iron filings or charcoal Almost any combination of an oxidizer and a fuel may be used along with the compounds needed to produce a desired special effect. Oxidizers Fuels (Reductants) Special Effects Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.