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Modern Atomic Theory

Modern Atomic Theory. Physical Science. State Standards. CLE.3202.Inq.1 – Recognize that science is a progressive endeavor that reevaluates and extends what is already accepted. Review of First Models. Democritus : Used atomos to describe matter

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Modern Atomic Theory

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  1. Modern Atomic Theory Physical Science

  2. State Standards • CLE.3202.Inq.1 – Recognize that science is a progressive endeavor that reevaluates and extends what is already accepted

  3. Review of First Models • Democritus : Used atomos to describe matter • No experiments, just observation with naked eye • John Dalton : Experiments to combine gases • Developed Law of Definite Proportions • J.J. Thompson : Cathode-Ray Tube • Discovered electrons / Plum Pudding model • Earnest Rutherford : Gold Foil experiment • Discovered nucleus ( protons ) / Modern view • Electrons orbit nucleus in definite paths ( planets/sun )

  4. What Do We Know Now? • Model ( conceptual view ) of the atom has changed significantly since Rutherford’s model • Now know that electrons are found only in ( at ) certain energy levels – not between levels • Electrons act like waves • Exact locations of electrons cannot be determined

  5. Electron Energy Levels • In 1913, Niels Bohr suggested that electrons can reside only at certain energy levels • Electrons must gain energy to move up • Electrons must lose energy to move down ELEVATOR MODEL

  6. Electrons – Particles and Waves • By 1925, Bohr’s model no longer explained all aspects of electron behavior • Electrons act like particles but they also act like waves too • Wave-Particle duality • Smaller the particle, the more it acts like a wave • Concept of electrons redefined • Imagine a wave vibrating on a string ( demo? )

  7. Wave-Particle Duality • In 1923, Louis de Broglie ( a French physicist ) made a hypothesis that led to a statement of the WAVE-PARTICLE DUALITY of nature. • It included the present theory of atomic structure • De Broglie used research by Albert Einsten and Max Planck to develop an equation relating mass and velocity of a particle to its wavelength

  8. Electron Energy Levels • Number of levels filled depends on electrons • specific to an element • Valence electrons are those on the outer level • Valence e- determine properties ( reactivity ) Energy Level 1 holds 2 e- Energy Level 2 holds 8 e- NUCLEUS Energy Level 3 holds 18 e- Energy Level 4 holds 32 e-

  9. Electron Energy Levels of Li • Lithium as example • 3 electrons in Li • 1 Valence electrons • In level that holds 8 Energy Level 1 holds 2 e- Energy Level 2 holds 1 e- NUCLEUS Energy Level 3 holds 0 e- Energy Level 4 holds 0 e-

  10. Location of Electrons • Imagine the moving propeller of an airplane • Now determine the location of any blade at any time • How would you do this? • Exact position of electrons also hard to calculate • Moving very fast always ( never stop or slow down ) • Orbitals ( regions where electrons might be ) are thus used to describe an approximate location • Different orbital SHAPEfordifferent orbital types

  11. Electron Orbitals • Four orbitals : s, p, d, and f • dand f orbitals are more complex • 5 possible d orbitals ( holds up to 10 electrons total ) • 7 possible f orbitals ( holds up to 14 electrons total )

  12. Energy Levels and Orbitals • 1st energy level has only s orbital • 2nd energy level has s and p orbitals • 3rd energy level has s, p, and d orbitals Energy Level 1 holds 2 e- s orbital ONLY Energy Level 2 holds 8 e- s and p orbitals Energy Level 3 holds 18 e- s, p and d orbitals Energy Level 4 holds 32 e- s, p, d, and f orbitals

  13. Energy Levels and Orbitals of Li • 1st energy level has only s orbital • 2 electrons • 2nd energy level has s and p orbitals • 1 electron Energy Level 1 holds 2 e- s orbital ONLY Energy Level 2 holds 1 e- s and p orbitals Only 2s orbital used Energy Level 3 holds 0 e- s, p and d orbitals Energy Level 4 holds 0 e- s, p, d, and f orbitals

  14. Energy Levels and Orbitals • Each energy level has a unique orbital layout • Orbitals in each energy level decide total number of electrons that can be at each energy level • Example – the 2nd energy level holds 8 e- • has an s ( holds 2 e-) and a p ( holds 6 e-)orbital

  15. Electron Transitions • Electron locations limited to certain energy levels • How do electrons move between levels? • When an atom gains or loses energy • Ground state – lowest energy level of electron • At normal (room) temps, electrons at ground state • Excited state – when electron has gained energy

  16. Atoms Emit & Absorb Light • Photons are absorbed by electrons to move it from ground to an excited state • Photon – a particle of light • Photons have different energies • Elevator model – energy of emitted photon depends on how many levels are crossed

  17. Atoms Emit & Absorb Light • Energy of photon is related to wavelength of light • High-energy photons have short wavelengths • Low-energy photons have long wavelengths • Wavelengths emitted depend on element • Since each element has a unique structure • Atomic “fingerprint” • Neon signs : Neon ( Ne ) gas produces red light • other gasesemit different colors of light • Color depends on wavelength of light emitted

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