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Atomic Structure

Atomic Structure. From Indivisible to Quantum Mechanical Model of the Atom. Classical Model. Democritus Dalton Thomson Rutherford. Democritus. Circa 400 BC Greek philosopher Suggested that all matter is composed of tiny, indivisible particles, called atoms.

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Atomic Structure

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  1. Atomic Structure From Indivisible to Quantum Mechanical Model of the Atom V.Montgomery & R.Smith

  2. Classical Model • Democritus • Dalton • Thomson • Rutherford V.Montgomery & R.Smith

  3. Democritus • Circa 400 BC • Greek philosopher • Suggested that all matter is composed of tiny, indivisible particles, called atoms V.Montgomery & R.Smith

  4. Dalton’s Atomic Theory (1808) • All matter is made of tiny indivisible particles called atoms. • Atoms of the same element are identical. The atoms of any one element are different from those of any other element. • Atoms of different elements can combine with one another in simple whole number ratios to form compounds. • Chemical reactions occur when atoms are separated, joined, or rearranged;however, atoms of one element are not changed into atoms of another by a chemical reaction. V.Montgomery & R.Smith

  5. J.J. Thomson (1897) • Determined the charge to mass ratio for electrons • Applied electric and magnetic fields to cathode rays • “Plum pudding” model of the atom V.Montgomery & R.Smith

  6. Rutherford’s Gold Foil Experiment (1910) • Alpha particles (positively charged helium ions) from a radioactive source was directed toward a very thin gold foil. • A fluorescent screen was placed behind the Au foil to detect the scattering of alpha () particles. V.Montgomery & R.Smith

  7. V.Montgomery & R.Smith

  8. Rutherford’s Gold Foil Experiment (Observations) • Most of the -particles passed through the foil. • Many of the -particles deflected at various angles. • Surprisingly, a few particles were deflected back from the Au foil. V.Montgomery & R.Smith

  9. Rutherford’s Gold Foil Experiment (Conclusions) • Rutherford concluded that most of the mass of an atom is concentrated in a core, called the atomic nucleus. • The nucleus is positively charged. • Most of the volume of the atom is empty space. V.Montgomery & R.Smith

  10. Shortfalls of Rutherford’s Model • Did not explain where the atom’s negatively charged electrons are located in the space surrounding its positively charged nucleus. • We know oppositely charged particles attract each other • What prevents the negative electrons from being drawn into the positive nucleus? V.Montgomery & R.Smith

  11. Bohr Model (1913) • Niels Bohr (1885-1962), Danish scientist working with Rutherford • Proposed that electrons must have enough energy to keep them in constant motion around the nucleus • Analogous to the motion of the planets orbiting the sun V.Montgomery & R.Smith

  12. Planetary Model • The planets are attracted to the sun by gravitational force, they move with enough energy to remain in stable orbits around the sun. • Electrons have energy of motion that enables them to overcome the attraction for the positive nucleus V.Montgomery & R.Smith

  13. Think about satellites…. • We launch a satellite into space with enough energy to orbit the earth • The amount of energy it is given, determines how high it will orbit • We use energy from a rocket to boost our satellite, what energy do we give electrons to boost them? V.Montgomery & R.Smith

  14. Electronic Structure of Atom • Waves-particle duality • Photoelectric effect • Planck’s constant • Bohr model • de Broglie equation V.Montgomery & R.Smith

  15. Radiant Energy • Radiation the emission of energy in various forms • A.K.A. Electromagnetic Radiation • Radiant Energy travels in the form of waves that have both electrical and magnetic impulses V.Montgomery & R.Smith

  16. Electromagnetic Radiation radiation that consists of wave-like electric and magnetic fields in space, including light, microwaves, radio signals, and x-rays • Electromagnetic waves can travel through empty space, at the speed of light (c=3.00x108m/s) or about 300million m/s!!! V.Montgomery & R.Smith

  17. Waves Waves transfer energy from one place to another Think about the damage done by waves during strong hurricanes. Think about placing a tennis ball in your bath tub, if you create waves at one it, that energy is transferred to the ball at the other = bobbing Electromagnetic waves have the same characteristics as other waves V.Montgomery & R.Smith

  18. Wave Characteristics Wavelength,  (lambda)  distance between successive points 2m 10m V.Montgomery & R.Smith

  19. Wave Characteristics • Frequency,  (nu)  the number of complete wave cycles to pass a given point per unit of time; Cycles per second t=5 t=0 t=0 t=5 V.Montgomery & R.Smith

  20. Units for Frequency • 1/s • s-1 • hertz, Hz • Because all electromagnetic waves travel at the speed of light, wavelength is determined by frequency • Low frequency = long wavelengths • High frequency = short wavelengths V.Montgomery & R.Smith

  21. Waves • Amplitude maximum height of a wave V.Montgomery & R.Smith

  22. Waves • Node points of zero amplitude V.Montgomery & R.Smith

  23. Electromagnetic Spectrum • Radio & TV, microwaves, UV, infrared, visible light = all are examples of electromagnetic radiation (and radiant energy) • Electromagnetic spectrum: entire range of electromagnetic radiation V.Montgomery & R.Smith

  24. Electromagnetic Spectrum Frequency Hz 1024 1020 1018 1016 1014 1012 1010 108 106 Gamma Xrays UV Microwaves FM AM IR 10-16 10-9 10-8 10-6 10-3 100 102 Wavelength m Visible Light V.Montgomery & R.Smith

  25. Notes • Higher-frequency electromagnetic waves have higher energy than lower-frequency electromagnetic waves • All forms of electromagnetic energy interact with matter, and the ability of these different waves to penetrate matter is a measure of the energy of the waves V.Montgomery & R.Smith

  26. What is your favorite radio station? • Radio stations are identified by their frequency in MHz. • We know all electromagnetic radiation(which includes radio waves) travel at the speed of light. • What is the wavelength of your favorite station? V.Montgomery & R.Smith

  27. Velocity of a Wave • Velocity of a wave (m/s) = wavelength (m) x frequency (1/s) • c =  • c= speed of light = 3.00x108 m/s • My favorite radio station is 105.9 Jamming Oldies!!! • What is the wavelength of this FM station? V.Montgomery & R.Smith

  28. Wavelength of FM • c =  • c= speed of light = 3.00x108 m/s •  = 105.9MHz or 1.059x108Hz •  = c/ =3.00x108 m/s = 2.83m 1.059x1081/s V.Montgomery & R.Smith

  29. What does the electromagnetic spectrum have to do with electrons? • It’s all related to energy – energy of motion(of electrons) and energy of light V.Montgomery & R.Smith

  30. States of Electrons • When current is passed through a gas at a low pressure, the potential energy (energy due to position) of some of the gas atoms increases. • Ground State: the lowest energy state of an atom • Excited State: a state in which the atom has a higher potential energy than it had in its ground state V.Montgomery & R.Smith

  31. Neon Signs • When an excited atom returns to its ground state it gives off the energy it gained in the form of electromagnetic radiation! • The glow of neon signs,is an example of this process V.Montgomery & R.Smith

  32. White Light • White light is composed of all of the colors of the spectrum = ROY G BIV • When white light is passed through a prism, the light is separated into a spectrum, of all the colors • What are rainbows? V.Montgomery & R.Smith

  33. Line-emission Spectrum • When an electric current is passed through a vacuum tube containing H2 gas at low pressure, and emission of a pinkish glow is observed. • What do you think happens when that pink glow is passed through a prism? V.Montgomery & R.Smith

  34. Hydrogen’s Emission Spectrum • The pink light consisted of just a few specific frequencies, not the whole range of colors as with white light • Scientists had expected to see a continuous range of frequencies of electromagnetic radiation, because the hydrogen atoms were excited by whatever amount of energy was added to them. • Lead to a new theory of the atom V.Montgomery & R.Smith

  35. Bohr’s Model of Hydrogen Atom • Hydrogen did not produce a continuous spectrum • New model was needed: • Electrons can circle the nucleus only in allowed paths or orbits • When an e- is in one of these orbits, the atom has a fixed, definite energy • e- and hydrogen atom are in its lowest energy state when it is in the orbit closest to the nucleus V.Montgomery & R.Smith

  36. Bohr Model Continued… • Orbits are separated by empty space, where e- cannot exist • Energy of e- increases as it moves to orbits farther and farther from the nucleus (Similar to a person climbing a ladder) V.Montgomery & R.Smith

  37. Bohr Model and Hydrogen Spectrum • While in orbit, e- can neither gain or lose energy • But, e- can gain energy equal to the difference between higher and lower orbitals, and therefore move to the higher orbital (Absorption) • When e- falls from higher state to lower state, energy is emitted (Emission) V.Montgomery & R.Smith

  38. Bohr’s Calculations • Based on the wavelengths of hydrogen’s line-emission spectrum, Bohr calculated the energies that an e- would have in the allowed energy levels for the hydrogen atom V.Montgomery & R.Smith

  39. Photoelectric Effect • An observed phenomenon, early 1900s • When light was shone on a metal, electrons were emitted from that metal • Light was known to be a form of energy, capable of knocking loose an electron from a metal • Therefore, light of any frequency could supply enough energy to eject an electron. V.Montgomery & R.Smith

  40. Photoelectric Effect pg. 93 • Light strikes the surface of a metal (cathode), and e- are ejected. • These ejected e- move from the cathode to the anode, and current flows in the cell. • A minimum frequency of light is used. If the frequency is above the minimum and the intensity of the light is increased, more e- are ejected. V.Montgomery & R.Smith

  41. Photoelectric Effect • Observed: For a given metal, no electrons were emitted if the light’s frequency was below a certain minimum, no matter how long the light was shone • Why does the light have to be of a minimum frequency? V.Montgomery & R.Smith

  42. Explanation…. • Max Planck studied the emission of light by hot objects • Proposed: objects emit energy in small, specific amounts = quanta (Differs from wave theory which would say objects emit electromagnetic radiation continuously) Quantum: is the minimum quantity of energy that can be lost or gained by an atom. V.Montgomery & R.Smith

  43. Planck’s Equation • E radiation = Planck’s constant x frequency of radiation • E = h • h = Planck’s constant = 6.626 x 10-34 J•s • When an object emits radiation, there must be a minimum quantity of energy that can be emitted at any given time. V.Montgomery & R.Smith

  44. Einstein Expands Planck’s Theory • Theorized that electromagnetic radiation had a dual wave-particle nature! • Behaves like waves and particles • Think of light as particles that each carry one quantum of energy = photons V.Montgomery & R.Smith

  45. Photons • Photons: a particle of electromagnetic radiation having zero mass and carrying a quantum of energy • Ephoton = h V.Montgomery & R.Smith

  46. Back to Photoelectric Effect • Einstein concluded: • Electromagnetic radiation is absorbed by matter only in whole numbers of photons • In order for an e- to be ejected, the e- must be struck by a single photon with minimum frequency V.Montgomery & R.Smith

  47. Example of Planck’s Equation • CD players use lasers that emit red light with a  of 685 nm. Calculate the energy of one photon. • Different metals require different minimum frequencies to exhibit photoelectric effect V.Montgomery & R.Smith

  48. Answer • Ephoton = h • h = Planck’s constant = 6.626 x 10-34 J•s • c =  • c= speed of light = 3.00x108 m/s • = (3.00x108 m/s)/(6.85x10-7m) • =4.37x10141/s • Ephoton= (6.626 x 10-34 J•s)(4.37x10141/s) Ephoton= 2.90 x 10-19J V.Montgomery & R.Smith

  49. Wave Nature of Electrons • We know electrons behave as particles • In 1925, Louis de Broglie suggested that electrons might also display wave properties V.Montgomery & R.Smith

  50. de Broglie’s Equation • A free e- of mass (m) moving with a velocity (v) should have an associated wavelength:  = h/mv • Linked particle properties (m and v) with a wave property () V.Montgomery & R.Smith

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