Quantum Theory, Part 1, Day 3

1. Quantum Theory, Part 1, Day 3

2. An unsatisfactory model for the hydrogen atom According to classical physics, light should be emitted as the electron circles the nucleus. A loss of energy would cause the electron to be drawn closer to the nucleus and eventually spiral into it. Hill, Petrucci, General Chemistry An Integrated Approach 2nd Edition, page 294

3. 1913 Niels Bohr • Danish • Planetary Atomic Model • Electrons are in energy levels (orbits) around the nucleus. • The electrons were attracted to the nucleus because of opposite charges. • Didn’t fall into the nucleus because it was moving around. • Energy levels have a definite diameter. • Energy increases further from the nucleus. • Energy can only occupy certain (discrete) levels • Energy absorbed = energy levels • Led to Spectroscopy • Electrons absorb energy and release it as color.

4. + Bohr’s Model of the Atom

6. Bohr’s Model Nucleus Electron Orbit Energy Levels

7. } Bohr’s Model Fifth • 7 Energy Levels • They… • Get bigger • Hold more e-’s • Have more energy • Get closer ….as you move away from the nucleus. • Further away from the nucleus means more energy. • There is no “in between” energy. • A quanta is the amount of energy needed to move from one energy level to another. • Since the energy of an atom is never “in between” there must be a quantum leap in energy. Fourth Third Increasing energy Second First Nucleus

8. The Bohr Ring Atom • He didn’t know why but only certain energies were allowed. • He called these allowed energies “energy levels.” • Putting Energy into the atom moved the electron away from the nucleus. • From ground state to excited state. • When it returns to ground state it gives off radiation (light) of a certain energy.

9. The Bohr Ring Atom n = 4 n = 3 n = 2 n = 1

10. Changing the energy • The energy level an electron starts from is called its ground state.

11. Changing the energy • Adding energy (heat, electricity, or light) can move the electron up energy levels • Electrons absorb the energy.

12. Changing the energy • The electron is now in its excited state. • As the electron falls back to ground state it gives the energy back, typically as light.

13. Changing the energy • May fall down in steps, each with a different energy.

14. { { {

15. What energies would exist? What would that mean we would see?

16. WHAT IS LIGHT?? • James Maxwell (1860) • Defined light as a form of kinetic energy (radiant) that travels through a vacuum (empty space) as electromagnetic waves. • Light is a form of electromagnetic radiation.

17. How are Electromagnetic Waves Created? • Charged particles (electrons) naturally generate ELECTRIC fields. • The movement of these charged particles creates a MAGNETIC field and thus, EM Radiation.

18. A wave has 4 characteristics… Units: m/s • Speed (c) Distance time • Light travels through a vacuum at • 2.998 x 108 m/s (approx. 186,282 miles/sec) THUS, LIGHT IS IDENTIFIED BY ITS SPEED!

19. 2. Amplitude Crest Baseline • Defined as the height of the wave from the baseline to a crest or a trough. Trough Node • Nodes - points of zero amplitude (wave crosses the axis). • Determines the intensity (brightness) of the radiation/light.

20. 3. Wavelength ( - lambda) -Distance between 2 consecutive crests or troughs. Units:m Angstroms (Å) = 1 x 10-10 m Determines the type of the radiation

21. 4. Frequency ( - nu) Defined as the number of crests that pass a given point in a unit of time (usually 1 sec). Units: 1/s, sec-1, or Hertz (Hz) Determines the type of the radiation

22. 0.01 sec. 500 1/s 0.06 sec. 50 1/s Light waves have frequencies from 100 to 1.0 x 1020 sec-1

23. Frequency = n

24. Frequency and Wavelength • Are Inversely related. • (As one increases, the other decreases.) • c = λν • Different frequencies of light are different colors of light. • There are a wide variety of frequencies. • The whole range is called a spectrum.

25. The Electromagnetic Spectrum • The electromagnetic spectrum is largely invisible to the eye • However, we can feel some radiation through other senses • Sunburned skin is a sign of too much ultraviolet radiation • Materials vary in their ability to absorb or transmit different parts of the electromagnetic spectrum • Our bodies absorb visible light, but transmit most X-rays • Window glass transmits visible light, but absorbs ultraviolet radiation

26. The Electromagnetic Spectrum

27. X-Rays Radiowaves Microwaves Ultra-violet GammaRays Infrared . Low Frequency High Frequency Long Wavelength Short Wavelength Visible Light

28. 1. Gamma Rays • Generated by the decay of radioactive atoms and in nuclear explosions. • Gamma-rays kill living cells • Can cause cancer, but can also be used to treat cancer patients

29. 2. X-Rays • The electrons are liberated from a hot cathode and accelerated by a high voltage towards the anode. • The X-rays are produced when the electrons collide with the atoms and nuclei of the Tungsten metal target. • Dangerous… can cause cancer • Used to examine bones  absorbed by bone but can travel through tissue easily. • Absorbed more readily by metal

30. 3. Ultraviolet “beyond violet” • Divided into three regions: • the near ultraviolet, • NUV, is the light closest to optical or visible light • the far ultraviolet, • FUV, lies between the near and extreme ultraviolet regions. It is the least explored of the three regions. • the extreme ultraviolet. • EUV, is the ultraviolet light closest to X-rays • Released from the sun and causes sunburn and photoaging • Dangerous– cancer euv

31. Ultraviolet Rays

32. 3. Ultraviolet “beyond violet” • UV rays are effective in killing bacteria and viruses. Hospitals use them to sterilize equipment, water, and air in operating rooms. • Used to kill microorganisms in air systems of large buildings • Produced by electrons moving within the atom. (Energy level 1) • Bees, bats, butterflies and some birds see UV light. (The reflection of UV rays off wings help insects identify mates).

33. 5. Infrared “below red” • The longer, far infrared wavelengths are about the size of a pin head and the shorter, near infrared ones are the size of cells, or are microscopic. • Released from objects with temps between (0K – 1000K) • Far infrared waves are thermal. • In other words, we experience this type of infrared radiation every day in the form of heat! The heat that we feel from sunlight, a fire, a radiator or a warm sidewalk is infrared. • Humans may not be able to see infrared light, but snakes in the pit viper family, like rattlesnakes, have sensory "pits", which are used to image infrared light.

34. Infrared light is even used to heat food sometimes - special lamps that emit thermal infrared waves are often used in fast food restaurants! • Shorter, near infrared waves are not hot at all - in fact you cannot even feel them. These shorter wavelengths are the ones used by your TV's remote control. • Produced by electrons moving within the atom. (Energy level = 3)

35. 6. Radar • Wavelengths are just a few inches long. • “Radio detection and ranging” • Produced by rotating molecules • During WWII, the British Government wanted a 'death-ray' weapon using radio waves. • At the time, Robert Watson-Watt was working for the National Physical Laboratory in Slough. • He did not invent a 'death-ray' weapon but he did find that his radio transmitters could create an echo from a plane that was over 200 miles away.

36. Work? • Emit radio waves which are reflected by the intended target. • By calculating the time, frequency, and direction change of the reflected wave (echo), they can determine the position & speed of the object.

37. Used For… • Enforce speed limits. • Astronomy • NEXRAD Doppler radar – calculate position & velocity of weather elements. • Burglar alarms • Planes • MRI (magnetic resonance imaging)

38. 6. Microwaves Microwaves have wavelengths that can be measured in centimeters! The longer microwaves, those closer to a foot in length, are the waves which heat our food in a microwave oven. WORKS? • Waves are absorbed by water, fat, and sugar • Causing the molecules to twist (rotate) – creating friction (Heat) RKE  MKE  TKE

39. What happens when metal goes in the microwave? • Microwaves cause electricity to flow in metals • Metals reflect microwaves • Thin metals – too much electricity flow (get Hot) • Sharp edges – electricity builds up and jumps (Sparks) WARNING: Putting metal in the microwave can RUIN the MAGNETRON.

40. Will water explode when heated in the microwave? • If it is superheated (heated beyond its BP) This can only be done if no bubbles are allowed to form. • Pure water • Reheated water • Water that sits out overnight

41. Will microwaves hurt us or the poodle? • Door closed - the wire mesh keeps microwaves in • Door open - Heats water in you (body usually distributes the heat evenly), but your eyes will suffer.

42. Why can we only see Visible Light? • Adapted to what we needed! • Gamma, X-ray, and UV too • damaging • IR – Low Frequency too • big

43. Types of Electromagnetic Radiation (Light): Visible – (What we consider light.) broken into colors ROY G. BIV VIBGYO R 400nm 700nm 7.50 x 1014 1/s 4.29 x 1014 1/s • The only  and  the receptors in your eyes/brain • can SEE.

44. Red • Long wavelength • Low frequency • Low energy • Violet • Short wavelength • High frequency • High energy

45. HOW DOES THE EYE WORK? • Light travels in a straight line. • The cornea focuses light on the retina. • The image appears upside down.

46. Your brain is so USED to seeing things upside-down that it eventually adjusts to it. After all, it's a lot easier to flip the image over than it is to try and coordinate your hands and legs with an upside-down world! As a result, though, it is believed that for the first few days, babies see everything upside-down. This is because they have not become used to vision. Your brain CAN be retrained though. In one psychological study, participants were asked to wear inverting lenses - lenses that invert the image BEFORE they get to your eye, so that when your eye inverts it, it's right-side-up. At first, everything appeared upside-down to the participants. But, after a few days, people began to report that everything appeared right-side-up! As a second part of the study, the people were asked to take the glasses off. Because they were now used to the lenses, their NORMAL vision appeared upside-down!! Within a day, though, their vision returned to normal. The reason you don't see everything upside-down, then, is simply because it's easier to think about right-side-up!

47. The retina contains 125 million photosensitive cells divided into 2 categories… RODS– respond to amplitude (see in dim light) • Night Blindness – lack Vitamin A CONES – respond to color • Red/Orange • Yellow/Green Color Blindness – sex linked • Blue/Purple • See over 10 million shades of color & use 1/5th of your brain.

48. The human eye is most sensitive to yellow- green, which is why more new fire engines are painted this color; it attracts attention easier. • Yellow- green is also easy to see at night because of their illuminating properties