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Electrons in Atoms

Chapter 5. Electrons in Atoms. What were early steps in development of atomic theory?. John Dalton – Billiard Ball Theory Atom was indivisible J.J. Thomson – Plum Pudding Model Atom was composed of smaller particles. Rutherford Model. n ucleus contains:

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Electrons in Atoms

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  1. Chapter 5 Electrons in Atoms

  2. What were early steps in development of atomic theory? • John Dalton – Billiard Ball Theory • Atom was indivisible • J.J. Thomson – Plum Pudding Model • Atom was composed of smaller particles

  3. Rutherford Model • nucleus contains: • all the positive charge & most of mass of atom • nucleus very small: • only 1/10,000th of atomic diameter • electrons occupy most of volume

  4. Later Models • Bohr – Planetary Model • Schrodinger – Wave Mechanical Model

  5. Problems with the Rutherford Model • Why don’t electrons crash into nucleus? • How are electrons arranged? • Why do different elements exhibit different chemical behavior? • How is atomic emission spectra produced?

  6. Atomic Emission Spectra • gas in glass tube & apply voltage across ends • produces light • color of light depends on gas in tube • every element produces its own unique color

  7. emission spectrum of element is set of frequencies (or wavelengths) emitted

  8. Why is emission spectra useful? • use it to determine if given element is present in sample • Neon lights

  9. Emission & Absorption Spectra of Elements

  10. Bohr Model • Bohr - electrons in atom can have only specific amounts of energy NEW idea! • each specific amount energy is associated with specific orbit • electrons restricted to these orbits • Bohr assigned quantum number (n) to each orbit • the smallest orbit (n= 1) • closest to nucleus • has lowest energy • larger the orbit, more energy it has

  11. E3 n=3 n=3 E2 n=2 E1 n=1 Bohr Diagram • Shows all the electrons in orbits or shells about the nucleus. n=2 n=1

  12. Bohr Model • energy absorbed when electron: • moves to higher orbit (farther from nucleus) • endothermic process • energy released when electron: • drops to lower orbit (closer to nucleus) • exothermic process

  13. energy levels get closer together the farther away they arefrom nucleus Larger orbits can hold more electrons

  14. Orbit Max # of Electrons 1 2 2 8 3 18 4 32 n 2n2 Max Capacity of Bohr Orbits

  15. Electron Transitions • If electron gains (absorbs) specific amount of energy • it can be excited to move to higher energy level • If electron loses specific amount of energy • it drops down to lower energy level

  16. Hydrogen has 1 electron, but it can make many possible electron transitions

  17. Absorption & Emission • cannot easily detect absorption of energy by electron BUT • can easily detect emission of energy by electron • photons (light) given off as excess energy is released

  18. Emitted Light • energy of emitted light (E = h • matches difference in energy between 2 levels • don’t know absolute energy of energy levels, but • observe light emitted due to energy changes

  19. ladder often used as analogy for energy levels of atom How is this one different? Potential Energy

  20. Ground State vs. Excited State • Ground state: • lowest energy state of atom • electrons in lowest possible energy levels • configurations in Reference Tables are ground state • Excited state: • many possible excited states for each atom • one or more electrons excited to higher energy level

  21. Success of Bohr’s Model • Bohr’s model could predict frequencies in emission spectrum of hydrogen • Predicted correct size of H atom • Unfortunately, didn’t work for anything with more than 1 electron

  22. Which principal energy level of an atom contains electron with the lowest energy? • n=1 • n=2 • n=3 • n=4

  23. What is total # of occupied principal energy levels in atom of neon in ground state? • 1 • 2 • 3 • 4

  24. What is total # of fully occupied principal energy levels in atom of nitrogen in ground state? • 1 • 2 • 3 • 4

  25. What is total # of electrons in completely filled fourth principal energy level? • 8 • 10 • 18 • 32

  26. Which atom in ground state has five electrons in its outer level and 10 electrons in its kernel? • C • Cl • Si • P

  27. Which electron configuration represents atom in excited state? a) 2-8-2 • 2-8-1 • 2-8 • 2-7-1

  28. Which electron configuration represents atom of Li in an excited state? • 1-1 • 1-2 • 2-1 • 2-2

  29. The characteristic bright-line spectrum of atom is produced by its • Electrons absorbing energy • Electrons emitting energy • Protons absorbing energy • Protons emitting energy

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